Erich Fromm 1961
The first hurdle to be cleared in order to arrive at a proper understanding of Marx's philosophy is the misunderstanding of the concept of materialism and historical materialism. Those who believe this to be a philosophy claiming that man's material interest, his wish for ever-increasing material gain and comforts, are his main motivation, forget the simple fact that the words "idealism" and "materialism" as used by Marx and all other philosophers have nothing to do with psychic motivations of a higher, spiritual level as against those of a lower and baser kind. In philosophical terminology, "materialism" (or "naturalism") refers to a philosophic view which holds that matter in motion is the fundamental constituent of the universe. In this sense the Greek pre-Socratic philosophers were "materialists," although they were by no means materialists in the abovementioned sense of the word as a value judgment or ethical principle. By idealism, on the contrary, a philosophy is understood in which it is not the everchanging world of the senses that constitutes reality, but incorporeal essences, or ideas. Plato's system is the first philosophical system to which the name of "idealism" was applied. While Marx was, in the philosophical sense a materialist in ontology, he was not even really interested in such questions, and hardly ever dealt with them.
However, there are many kinds of materialist and idealist philosophies, and in order to understand Marx's "materialism" we have to go beyond the general definition just given. Marx actually took a firm position against a philosophical materialism which was current among many of the most progressive thinkers (especially natural scientists) of his time. This materialism claimed that "the" substratum of all mental and spiritual phenomena was to be found in matter and material processes. In its most vulgar and superficial form, this kind of materialism taught that feelings and ideas are sufficiently explained as results of chemical bodily processes, and "thought is to the brain what urine is to the kidneys."
Marx fought this type of mechanical, "bourgeois" materialism "the abstract materialism of natural science, that excludes history and its process,"  and postulated instead what he called in the Economic and Philosophical Manuscripts "naturalism or humanism [which] is distinguished from both idealism and materialism, and at the same time constitutes their unifying truth."  In fact, Marx never used the terms "historical materialism" or "dialectic materialism"; he did speak of his own "dialectical method" in contrast with that of Hegel and of its "materialistic basis," by which he simply referred to the fundamental conditions of human existence.
This aspect of "materialism," Marx's "materialist method," which distinguishes his view from that of Hegel, involves the study of the real economic and social life of man and of the influence of man's actual way of life on this thinking and feeling. "In direct contrast to German philosophy," Marx wrote, "which descends from heaven to earth, here we ascend from earth to heaven. That is to say, we do not set out from what men imagine, conceive, nor from men as narrated, thought of, or imagined, conceived, in order to arrive at men in the flesh. We set out from real, active men and on the basis of their real life process we demonstrate the development of the ideological reflexes and echoes of this life process."  Or, as he puts it in a slightly different way: "Hegel's philosophy of history is nothing but the philosophical expression of the Christian-Germanic dogma concerning the contradiction between spirit and matter, God and the world.... Hegel's philosophy of history presupposes an abstract or absolute spirit, which develops in such a way that mankind is only a mass which carries this spirit, consciously or unconsciously. Hegel assumes that a speculative, esoterical history precedes and underlies empirical history. The history of mankind is transformed into the history of the abstract spirit of mankind, which transcends the real man." 
Marx described his own historical method very succinctly: "The way in which men produce their means of subsistence depends first of all on the nature of the actual means they find in existence and have to reproduce. This mode of production must not be considered simply as being the reproduction of the physical existence of the individuals. Rather, it is a definite form of activity of these individuals, a definite form of expressing their life, a definite mode of life on their part. As individuals express their life, so they are. What they are, therefore, coincides with their production, both with what they produce and with how they produce. The nature of individuals thus depends on the material conditions determining their production." 
Marx made the difference between historical materialism and contemporary materialism very clear in his thesis on Feuerbach: "The chief defect of all materialism up to now (including Feuerbach's) is that the object, reality, what we apprehend through our senses, is understood only in the form of the object or contemplation (Anschauung); but not as sensuous human activity, as practice; not subjectively. Hence in opposition to materialism, the active side was developed abstractly by idealism -- which of course does not know real sensuous activity as such. Feuerbach wants sensuous objects really distinguished from the objects of thought; but he does not understand human activity itself as objective activity."  Marx -- like Hegel -- looks at an object in its movement, in its becoming, and not as a static "object," which can be explained by discovering the physical "cause" of it. In contrast to Hegel, Marx studies man and history by beginning with the real man and the economic and social conditions under which he must live, and not primarily with his ideas. Marx was as far from bourgeois materialism as he was from Hegel's idealism -- hence he could rightly say that his philosophy is neither idealism nor materialism but a synthesis: humanism and naturalism.
It should be clear by now why the popular idea of the nature of historical materialism is erroneous. The popular view assumes that in Marx's opinion the strongest psychological motive in man is to gain money and to have more material comfort; if this is the main force within man, so continues this "interpretation" of historical materialism, the key to the understanding of history is the material desires of men; hence, the key to the explanation of history is man's belly, and his greed for material satisfaction. The fundamental misunderstanding on which this interpretation rests is the assumption that historical materialism is a psychological theory which deals with man's drives and passions. But, in fact, historical materialism is not at all a psychological theory; it claims that the way man produces determines his thinking and his desires, and not that his main desires are those for maximal material gain. Economy in this context refers not to a psychic drive, but to the mode of production; not to a subjective, psychological, but to an objective, economic-sociological factor. The only quasi-psychological premise in the theory lies in the assumption that man needs food, shelter, etc., hence needs to produce; hence that the mode of production, which depends on a number of objective factors, comes first, as it were, and determines the other spheres of his activities. The objectively given conditions which determine the mode of production and hence social organization, determine man, his ideas as well as his interests. In fact, the idea that "institutions form men," as Montesquieu put it, was an old insight; what was new in Marx was his detailed analysis of institutions as being rooted in the mode of production and the productive forces underlying it. Certain economic conditions, like those of capitalism, produce as a chief incentive the desire for money and property; other economic conditions can produce exactly the opposite desires, like those of asceticism and contempt for earthly riches, as we find them in many Eastern cultures and in the early stages of capitalism.  The passion for money and property, according to Marx, is just as much economically conditioned as the opposite passions. 
Marx's "materialistic" or "economic" interpretation of history has nothing whatsoever to do with an alleged "materialistic" or "economic" striving as the most fundamental drive in man. It does mean that man, the real and total man, the "real living individuals" -- not the ideas produced by these "individuals" -- are the subject matter of history and of the understanding of its laws. Marx's interpretation of history could be called an anthropological interpretation of history, if one wanted to avoid the ambiguities of the words "materialistic" and "economic"; it is the understanding of history based on the fact that men are "the authors and actors of their history." , 
In fact, it is one of the great differences between Marx and most writers of the eighteenth and nineteenth centuries that he does not consider capitalism to be the outcome of human nature and the motivation of man in capitalism to be the universal motivation within man. The absurdity of the view that Marx thought the drive for maximal profit was the deepest motive in man becomes all the more apparent when one takes into account that Marx made some very direct statements about human drives. He differentiated between constant or "fixed" drives "which exist under all circumstances and which can be changed by social conditions only as far as form and direction are concerned" and "relative" drives which "owe their origin only to a certain type of social organization." Marx assumed sex and hunger to fall under the category of "fixed" drives, but it never occurred to him to consider the drive for maximal economic gain as a constant drive. 
But it hardly needs such proof from Marx's psychological ideas to show that the popular assumption about Marx's materialism is utterly wrong. Marx's whole criticism of capitalism is exactly that it has made interest in money and material gain the main motive in man, and his concept of socialism is precisely that of a society in which this material interest would cease to be the dominant one. This will be even clearer later on when we discuss Marx's concept of human emancipation and of freedom in detail.
As I emphasized before, Marx starts out with man, who makes his own history: "The first premise of all human history is, of course, the existence of living human individuals. Thus the first fact to be established is the physical organization of these individuals and their consequent relation to the rest of nature. Of course, we cannot here go either into the actual physical nature of man, or into the natural conditions in which man finds himself -- geological, orohydrographical, climatic and so on. The writing of history must always set out from these natural bases and their modification in the course of history through the action of man. Men can be distinguished from animals by consciousness, by religion or anything else you like. They themselves begin to produce their means of subsistence, a step which is conditioned by their physical organization. By producing their means of subsistence men are indirectly producing their actual material life." 
It is very important to understand Marx's fundamental idea: man makes his own history; he is his own creator. As he put it many years later in Capital: "And would not such a history be easier to compile since, as Vico says, human history differs from natural history in this, that we have made the former, but not the latter."  Man gives birth to himself in the process of history. The essential factor in this process of self-creation of the human race lies in its relationship to nature. Man, at the beginning of his history, is blindly bound or chained to nature. In the process of evolution he transforms his relationship to nature, and hence himself.
Marx has more to say in Capital about this dependence on nature: "Those ancient social organisms of production are, as compared with bourgeois society, extremely simple and transparent. But they are founded either on the immature development of man individually, who has not yet severed the umbilical cord that unites him with his fellow men in a primitive tribal community, or upon direct relations of subjection. They can arise and exist only when the development of the productive power of labor has not risen beyond a low stage, and when, therefore, the social relations within the sphere of material life, between man and man, and between man and nature, are correspondingly narrow. This narrowness is reflected in the ancient worship of Nature, and in the other elements of the popular religions. The religious reflex of the real world can, in any case, only then finally vanish when the practical relations of everyday life offer to man none but perfectly intelligible and reasonable relations with regard to his fellow men and to nature. The life-process of society, which is based on the process of material production, does not strip off its mystical veil until it is treated as production by freely associated men, and is consciously regulated by them in accordance with a settled plan. This, however, demands for society a certain material groundwork or set of conditions of existence which in their turn are the spontaneous product of a long and painful process of development." 
In this statement Marx speaks of an element which has a central role in his theory: labor. Labor is the factor which meditates between man and nature; labor is man's effort to regulate his metabolism with nature. Labor is the expression of human life and through labor man's relationship to nature is changed, hence through labor man changes himself. More about his concept of labor will be said later on.
I will conclude this section by quoting Marx's most complete formulation of the concept of historical materialism, written in 1859:
"The general result at which I arrived and which, once won, served as a guiding thread for my studies, can be briefly formulated as follows: in the social production of their life, men enter into definite relations that are indispensable and independent of their will, relations of production which correspond to a definite stage of development of their material productive forces. The sum total of these relations of production constitutes the economic structure of society, the real foundation, on which rises a legal and political superstructure and to which correspond definite forms of social consciousness. The mode of production of material life conditions the social, political and intellectual life process in general. It is not the consciousness of men that determines their social being, but, on the contrary, their social being that determines their consciousness. At a certain stage of their development, the material productive forces of society come in conflict with the existing relations of production, or -- what is but a legal expression for the same thing -- with the property relations within which they have been at work hitherto. From forms of development of the productive forces these relations turn into their fetters. Then begins an epoch of social revolution. With the change of the economic foundations the entire immense superstructure is more or less rapidly transformed. In considering such transformations a distinction should always be made between the material transformation of the economic conditions of production, which can be determined with the precision of natural science, and the legal, political, religious, esthetic or philosophic -- in short, ideological forms in which men become conscious of this conflict and fight it out. Just as our opinion of an individual is not based on what he thinks of himself, so we cannot judge of such a period of transformation by its own consciousness; on the contrary, this consciousness must be explained rather from the contradictions of material life, from the existing conflict between the social productive forces and the relations of production. No social order ever perishes before all the productive forces for which there is room in it have developed; and new, higher relations of production never appear before the material conditions of their existence have matured in the womb of the old society itself. Therefore mankind always sets itself only such tasks as it can solve; since, looking at the matter more closely, it will always be found that the task itself arises only when the material conditions for its solution already exist or are at least in the process of formation. In broad outlines Asiatic, ancient, feudal, and modern bourgeois modes of production can be designated as progressive epochs in the economic formation of society. The bourgeois relations of production are the last antagonistic form of the social process of production -- antagonistic not in the sense of individual antagonism, but of one arising from the social conditions of life of the individual; at the same time the productive forces developing in the womb of bourgeois society create the material conditions for the solution of that antagonism. This social formation brings, therefore, the prehistory of human society to a close." 
It will be useful again to underscore and elaborate on some specific notions in this theory. First of all, Marx's concept of historical change. Change is due to the contradiction between the productive forces (and other objectively given conditions) and the existing social organization. When a mode of production or social organization hampers, rather than furthers, the given productive forces, a society, if it is not to collapse, will choose such forms of production as fit the new set of productive forces and develop them. The evolution of man, in all history, is characterized by man's struggle with nature. At one point of history (and according to Marx in the near future), man will have developed the productive sources of nature to such an extent that the antagonism between man and nature can be eventually solved. At this point "the prehistory of man" will come to a close and truly human history will begin.