Below, you’ll find the paragraphs in Karl Popper’s 1972 book Objective knowledge: an evolutionary approach, in which he introduces his theory of three worlds.

Knowledge in the Objective Sense

The commonsense theory of knowledge, and with it all -or almost all-philosophers until at least Balzano and Frege, took it for granted that there was only one kind of knowledge possessed by some knowing subject.

I will call this kind of knowledge ‘subjective knowledge‘, in spite of the fact that, as we shall see, genuine or unadulterated or purely subjective conscious knowledge simply does not exist. The theory of subjective knowledge is very old; but it becomes explicit with Descartes: ‘knowing’ is an activity and presupposes the existence of a knowing subject. It is the subjective self who knows.

Now I wish to distinguish between two kinds of ‘knowledge’: subjective knowledge (which should better be called organismic knowledge, since it consists of the dispositions of organisms); and objective knowledge, or knowledge in the objective sense, which consists of the logical content of our theories, conjectures, guesses (and, if we like, of the logical content of our genetic code).

Examples of objective knowledge are theories published in journals and books and stored in libraries; discussions of such theories; difficulties or problems pointed out in connection with such theories and so on.

We can call the physical world ‘world 1 ‘, the world of our conscious experiences ‘world 2’, and the world of the logical contents of books, libraries, computer memories, and suchlike ‘world 3’.

About this world 3 I have several theses:

(1) We can discover new problems in world 3 which were there before they were discovered and before they ever became conscious; that is, before anything corresponding to them appeared in world 2. Example: we discover prime numbers, and Euclid’s problem of whether the sequence of prime numbers is infinite arises as a consequence.

(2) Thus there is a sense in which world 3 is autonomous: in this world we can make theoretical discoveries in a similar way to that in which we can make geographical discoveries in world I.

(3) Main thesis: our conscious subjective knowledge (world 2 knowledge) depends upon world 3, that is to say on (at least virtually) linguistically formulated theories. Example: our ‘immediate self-consciousness’, or our ‘knowledge of self’, which is very important, depends very largely upon world 3 theories: on our theories about our body and its continued existence when we fall asleep or become unconscious; on our theories of time (its linearity); on our theory that we can pick up our memory of past experiences in various degrees of clarity; and so on. With these theories are connected our expectations of waking up after falling asleep. I propose the thesis that full consciousness of self depends upon all these (world 3) theories, and that animals, although capable of feelings, sensations, memory, and thus of consciousness, do not possess the full consciousness of self which is one of the results of human language and the development of the specifically human world 3.

From: Karl Popper: Objective knowledge: An evolutionary approach (1972),
p. 73-4.