At the basis of Proudhon’s economic theory we find two essential ideas, that of value and that of exchange.
These two ideas are only of interest in the regime of individual property. in a communist society, in fact, one does not produce in order to sell, but to consume; the question of the exchange value of objects for consumption is thus no longer posed, as there is no longer exchange (sale), but simple distribution. Consequently, the problem that concerned Proudhon so much, that of the “constitution of value,” does not exist where social products are produced by a social labor, destined to be consumed by the community of producers.
In his Economic Contradictions (1846) Proudhon dedicated a chapter to the “constitution of value.” Here are the principle passages:
The economists seem always to have understood by the measure of value only a standard, a sort of original unit, existing by itself, and applicable to all sorts of merchandise, as the yard is applicable to all lengths. Consequently, many have thought that such a standard is furnished by the precious metals. But the theory of money has proved that, far from being the measure of values, specie is only their arithmetic, and a conventional arithmetic at that. …
The idea that has been entertained hitherto of the measure of value, then, is inexact; the object of our inquiry is not the standard of value, as has been said so often and so foolishly, but the law which regulates the proportions of the various products to the social wealth; for upon the knowledge of this law depends the rise and fall of prices in so far as it is normal and legitimate. In a word, as we understand by the measure of celestial bodies the relation resulting from the comparison of these bodies with each other, so, by the measure of values, we must understand the relation which results from their comparison. Now, I say that this relation has its law, and this comparison its principle.
I suppose, then, a force which combines in certain proportions the elements of wealth, and makes of them a homogeneous whole: if the constituent elements do not exist in the desired proportion, the combination will take place nevertheless; but, instead of absorbing all the material, it will reject a portion as useless. The internal movement by which the combination is produced, and which the affinities of the various substances determine — this movement in society is exchange; exchange considered no longer simply in its elementary form and between man and man, but exchange considered as the fusion of all values produced by private industry in one and the same mass of social wealth. Finally, the proportion in which each element enters into the compound is what we call value; the excess remaining after the combination is non-value, until the addition of a certain quantity of other elements causes further combination and exchange.
Although having placed himself on the terrain of private production, Proudhon regarded the products, once entered into the general consumption, as having received a social character. It is, he said, exchange which, by merging all the values produced by private industries into a single social wealth, impresses them with that character. Only, the products exchanged, those that have become social, or, in other words, entered into the combination by which the products become social wealth, have a true value; those of the products that cannot be absorbed, that is, exchanged and consumed, remain, from the social point of view, non-values. There is, however, in this way of viewing the facts, something unsatisfying for the mind: it is that the products created by private industry, without preconceived plan, appear at the beginning as so many isolated objects, manufactured arbitrarily and by chance; but we see Proudhon correct himself, further along, this defect in his conception.
Let’s continue the quotation:
This determined, it is conceivable that at a given moment the proportions of values constituting the wealth of a country may be determined, or at least empirically approximated, by means of statistics and inventories, in nearly the same way that the chemists have discovered by experience, aided by analysis, the proportions of hydrogen and oxygen necessary to the formation of water. There is nothing objectionable in this method of determining values; it is, after all, only a matter of accounts. But such a work, however interesting it might be, would teach us nothing very useful. On the one hand, indeed, we know that the proportion continually varies; on the other, it is clear that from a statement of the public wealth giving the proportions of values only for the time and place when and where the statistics should be gathered we could not deduce the law of proportionality of wealth…
… Social economy, on the contrary, to which no a posteriori investigation could reveal directly the law of proportionality of values, can grasp it in the very force which produces it, and which it is time to announce.
This force, which Adam Smith has glorified so eloquently, and which his successors have misconceived (making privilege its equal), — this force is Labor…
Society, or the collective man, produces an infinitude of objects, the enjoyment of which constitutes its well-being. This well-being is developed not only in the ratio of the quantity of the products, but also in the ratio of their variety (quality) and proportion. From this fundamental datum it follows that society always, at each instant of its life, must strive for such proportion in its products as will give the greatest amount of well-being, considering the power and means of production. Abundance, variety, and proportion in products are the three factors which constitute Wealth…
But how establish this marvelous proportion, so essential that without it a portion of human labor is lost, — that is, useless, inharmonious, untrue, and consequently synonymous with poverty and annihilation?
Prometheus, according to the fable, is the symbol of human activity. Prometheus steals the fire of heaven, and invents the early arts; Prometheus foresees the future, and aspires to equality with Jupiter; Prometheus is God. Then let us call society Prometheus.
Prometheus knows that such a product costs an hour’s labor, such another a day’s, a week’s, a year’s; he knows at the same time that all these products, arranged according to their cost, form the progression of his wealth. First, then, he will assure his existence by providing himself with the least costly, and consequently most necessary, things; then, as fast as his position becomes secure, he will look forward to articles of luxury, proceeding always, if he is wise, according to the natural position of each article in the scale of prices. Sometimes Prometheus will make a mistake in his calculations, or else, carried away by passion, he will sacrifice an immediate good to a premature enjoyment, and, after having toiled and moiled, he will starve. Thus, the law carries with it its own sanction; its violation is inevitably accompanied by the immediate punishment of the transgressor.
According to this analysis, value, considered from the point of view of the association which producers, by division of labor and by exchange, naturally form among themselves, is the proportional relation of the products which constitute wealth; and what we call the value of any special product is a formula which expresses, in terms of money, the proportion of this product to the general wealth…
In the course of that explanation, the original point of view is transformed. Proudhon, who showed us, in the beginning, individual producers each working as they wish, without concert, so that part of the products risk remained unused, has substituted for private industry an entirely different conception of production. He no longer speaks to us of isolated laborers, but of the “society,” the “collective man;” he symbolized that collective man in the mythological character of Prometheus, “the one who foresees:” it is Prometheus who rules production by proportioning to the various needs. The producers, he said, “naturally form a society among themselves,” and there is the recognition of that truth, that production is also a social fact, and not only exchange. Proudhon shows a society that combines the efforts of labor in a manner to realize “such proportion in its products as will give the greatest amount of well-being;” which obtains “abundance, variety and proportion in the products;” a society, consequently, where the laborers are united and act in concert.
But that is the communist society.
And then, since production is social, since it is organized in advanced and proportional to needs, it is not, as Proudhon said at the beginning, by exchange that the socialization of products occurs. There is no need to socialize them after the fact; they are socialized in advance by the fact of the agreement and solidarity between the producers. The products once created, and created according to the quantity and proportion which have been settled upon by Prometheus, that is to say by society, it is not a question of exchanging them by of dividing them in conformity with the plan that has directed production, since the production has been done precisely with an eye to that division destined to satisfy the needs of all the producers.
Proudhon ended up, then, at the communist idea—although, in his horror of authority, he had battled energetically as we know the communists in the manner of Cabet or Blanqui.
Proudhon was an anti-authoritarian, anti-statist communist, a federalist communist. Was he a communist without knowing it?
No, he suspected it, and he said it, at least once.
In his famous letter to Marx, May 17, 1846 (published for the first time in the Correspondence), he wrote :
I myself put the problem in this way: to bring about the return to society, by an economic combination, of the wealth which was withdrawn from society by another economic combination. In other words, through Political Economy to turn the theory of Property against Property in such a way as to engender what you German socialists call community and what I will limit myself for the moment to calling liberty or equality.
Liberty and equality: that is how Proudhon formulated his social ideal; and that is the same, he said, as what Marx and his friends call community. 
We must believe him.
La Vie ouvrière, n°46-47 (August 20-September 5, 1911): 307-312.
[Working translation by Shawn P. Wilbur, with passages from The System of Economic Contradictions taken from Benjamin R. Tucker’s edition.]
 Proudhon’s book against property (What is Property?) was the first socialist expression, of historical importance, which issued from the proletariat. Marx knew that book before coming to Paris, and it was like a revelation for him (eine Art Offenbarung)… The writing of Proudhon impressed him, by its careful and substantial language, its penetrating dissection of jurisprudence and bourgeois political economy by the boldness with which it subjected property to a critical analysis, and, especially, because that writing was the work of a proletarian. (Franz Mehring, Gesammelte Schriften von Karl Marx und Friedrich Engels, II, pages 11-12, 1902.)