Thinking is an act which idealizes experience by transforming sensations into an intelligible whole. It works by seizing upon the ideal element which is already there, conserving it, and developing it. It produces knowledge by supplying relations to experience. Dewey realizes that his act of intelligence is similar to Kant's 'apperceptive unity.' He says: "The mention of Kant's name suggests that both his strength and his weakness lie in the line just mentioned. It is his strength that he recognizes that an apperceptive unity interpreting sensations through categories which constitute the synthetic content of self-consciousness is indispensable to experience. It is his weakness that he conceives this content as purely logical, and hence as formal."[34] Kant's error was to treat the self as formal and held apart from its material. "The self does not work with _a priori_ forms upon an _a posteriori_ material, but intelligence as ideal (or _a priori_) constitutes experience (or the _a posteriori_) as having meaning."[35] Dewey's standpoint here seems to be similar to that of Green. But as Kant's unity of apperception became for Green merely a symbol of the world's inherent intelligibility, the latter did not regard it as an actual process of synthesis. Dewey fails to make a distinction, which might have been useful to him, between Kant's unity of apperception and his productive imagination. It is the latter which Dewey retains, and he tends to identify it with the empirical process of the understanding. Knowing, psychologically considered, is a synthetic process. "And this is to say that experience grows as intelligence adds out of its own ideal content ideal quality.... The growth of the power of comparison implies not a formal growth, but a synthetic internal growth."[36] Dewey, of course, views understanding as an integral part of reality's processes rather than as a process apart, but it is for him a very special activity, which builds up the meaning of experience. "Knowledge might be indifferently described, therefore, as a process of idealisation of experience, or of realisation of intelligence. It is each through the other. Ultimately the growth of experience must consist in the development out of itself by intelligence of its own implicit ideal content upon occasion of the solicitation of sensation."[37]