The concept of the active, productive man who grasps and embraces the objective world with his own powers cannot be fully understood without the concept of the negation of productivity: alienation. For Marx the history of mankind is a history of the increasing development of man, and at the same time of increasing alienation. His concept of socialism is the emancipation from alienation, the return of man to himself, his self-realization.
Alienation (or "estrangement") means, for Marx, that man does not experience himself as the acting agent in his grasp of the world, but that the world (nature, others, and he himself) remain alien to him. They stand above and against him as objects, even though they may be objects of his own creation. Alienation is essentially experiencing the world and oneself passively, receptively, as the subject separated from the object.
The whole concept of alienation found its first expression in Western thought in the Old Testament concept of idolatry. The essence of what the prophets call "idolatry" is not that man worships many gods instead of only one. It is that the idols are the work of man's own hands -- they are things, and man bows down and worships things; worships that which he has created himself. In doing so he transforms himself into a thing. He transfers to the things of his creation the attributes of his own life, and instead of experiencing himself as the creating person, he is in touch with himself only by the worship of the idol. He has become estranged from his own life forces, from the wealth of his own potentialties, and is in touch with himself only in the indirect way of submission to life frozen in the idols. The deadness and emptiness of the idol is expressed in the Old Testament: "Eyes they have and they do not see, ears they have and they do not hear," etc. The more man transfers his own powers to the idols, the poorer he himself becomes, and the more dependent on the idols, so that they permit him to redeem a small part of what was originally his. The idols can be a godlike figure, the state, the church, a person, possessions. Idolatry changes its objects; it is by no means to be found only in those forms in which the idol has a socalled religious meaning. Idolatry is always the worship of something into which man has put his own creative powers, and to which he now submits, instead of experiencing himself in his creative act. Among the many forms of alienation, the most frequent one is alienation in language. If I express a feeling with a word, let us say, if I say "I love you," the word is meant to be an indication of the reality which exists within myself, the power of my loving. The word "love" is meant to be a symbol of the fact love, but as soon as it is spoken it tends to assume a life of its own, it becomes a reality. I am under the illusion that the saying of the word is the equivalent of the experience, and soon I say the word and feel nothing, except the thought of love which the word expresses. The alienation of language shows the whole complexity of alienation. Language is one of the most precious human achievements; to avoid alienation by not speaking would be foolish -- yet one must be always aware of the danger of the spoken word, that it threatens to substitute itself for the living experience. The same holds true for all other achievements of man; ideas, art, any kind of man-made objects. They are man's creations; they are valuable aids for life, yet each one of them is also a trap, a temptation to confuse life with things, experience with artifacts, feeling with surrender and submission.
The thinkers of the eighteenth and nineteenth centuries criticized their age for its increasing rigidity, emptiness, and deadness. In Goethe's thinking the very same concept of productivity that is central in Spinoza as well as in Hegel and Marx, was a cornerstone. "The divine," he says, "is effective in that which is alive, but not in that which is dead. It is in that which is becoming and evolving, but not in that which is completed and rigid. That is why reason, in its tendency toward the divine, deals only with that which is becoming, and which is alive, while the intellect deals with that which is completed and rigid, in order to use it." 
We find similar criticisms in Schiller and Fichte, and then in Hegel and in Marx, who makes a general criticism that in his time "truth is without passion, and passion is without truth." 
Essentially the whole existentialist philosophy, from Kierkegaard on, is, as Paul Tillich puts it, "an over onehundred-years-old movement of rebellion against the dehumanization of man in industrial society." Actually, the concept of alienation is, in nontheistic language, the equivalent of what in theistic language would be called "sin": man's relinquishment of himself, of God within himself. The thinker who coined the concept of alienation was Hegel. To him the history of man was at the same time the history of man's alienation (Entfremdung). "What the mind really strives for," he wrote in The Philosophy of History, "is the realization of its notion; but in doing so it hides that goal from its own vision and is proud and well satisfied in this alienation from its own essence."  For Marx, as for Hegel, the concept of alienation is based on the distinction between existence and essence, on the fact that man's existence is alienated from his essence, that in reality he is not what he potentially is, or, to put it differently, that he is not what he ought to be, and that he ought to be that which he could be.
For Marx the process of alienation is expressed in work and in the division of labor. Work is for him the active relatedness of man to nature, the creation of a new world, including the creation of man himself. (Intellectual activity is of course, for Marx, always work, like manual or artistic activity.) But as private property and the division of labor develop, labor loses its character of being an expression of man's powers; labor and its products assume an existence separate from man, his will and his planning. "The object produced by labor, its product, now stands opposed to it as an alien being, as a power independent of the producer. The product of labor is labor which has been embodied in an object and turned into a physical thing; this product is an objectification of labor."  Labor is alienated because the work has ceased to be a part of the worker's nature and "consequently, he does not fulfill himself in his work but denies himself, has a feeling of misery rather than well-being, does not develop freely his mental and physical energies but is physically exhausted and mentally debased. The worker therefore feels himself at home only during his leisure time, whereas at work he feels homeless."  Thus, in the act of production the relationship of the worker to his own activity is experienced "as something alien and not belonging to him, activity as suffering (passivity), strength as powerlessness, creation as emasculation."  While man thus becomes alienated from himself, the product of labor becomes "an alien object which dominates him. This relationship is at the same time the relationship to the sensuous external world, to natural objects, as an alien and hostile world."  Marx stresses two points: 1) in the process of work, and especially of work under the conditions of capitalism, man is estranged from his own creative powers, and 2) the .objects of his own work become alien beings, and eventually rule over him, become powers independent of the producer. "The laborer exists for the process of production, and not the process of production for the laborer."  ...