2.01 The state of affairs is a combination of objects (items, things).
This raises the question of what these objects are, although Wittgenstein's use of different words to name the same things perhaps implies that the meaning of "objects" is not well defined. If he meant something precise, one might think, why not stick to one carefully defined term?
According to Goldfarb (in Reck, ed., From Frege to Wittgenstein), so far as the TLP is a response to Russell, it is mostly a response to Part I of Principles of Mathematics, published in 1903 and mostly written in 1900.
In the preface, written in 1902, Russell writes: “The discussion of indefinables—which forms the chief part of philosophical logic—is the endeavour to see clearly, and to make others see clearly, the entities concerned, in order that the mind may have that kind of acquaintance with them which it has with redness or the taste of a pineapple. Where, as in the present case, the indefinables are obtained primarily as the necessary residue in a process of analysis, it is often easier to know that there must be such entities than actually to perceive them; there is a process analogous to that which resulted in the discovery of Neptune, with the difference that the final stage—the search with a mental telescope for the entity which has been inferred—is often the most difficult part of the undertaking.” (p. xv)
“Whatever may be an object of thought, or may occur in any true or false proposition, or can be counted as one, I call a term. This, then, is the widest word in the philosophical vocabulary. I shall use as synonymous with it the words unit, individual, and entity. … A man, a moment, a number, a class, a relation, a chimaera, or anything else that can be mentioned, is sure to be a term; and to deny that such and such a thing is a term must always be false.” (p. 43)
“A term is, in fact, possessed of all the properties commonly assigned to substances or substantives. Every term, to begin with, is a logical subject: it is, for example, the subject of the proposition that itself is one. Again every term is immutable and indestructible. What a term is, it is, and no change can be conceived in it which would not destroy its identity and make it another term. Another mark which belongs to terms is numerical identity with themselves and numerical diversity from all other terms.” (p. 44)
“Among terms it is possible to distinguish two kinds, which I shall call respectively things and concepts. The former are the terms indicated by proper names, the latter those indicated by all other words.” (p. 44)
Wittgenstein's giving different words for thing here suggests that the idea is meant to be intuitive or non-technical. What matters is the combination, the combinatory nature of states of affairs. See elsewhere on the concept of a thing, how its meaning could cover almost anything, and in a way that Kant at least considered to be problematic.
Wittgenstein later said that relations count as objects. This is in Desmond Lee (ed.) Wittgenstein's Lectures, Cambridge, 1930-32. Lee records Wittgenstein as saying that: "Objects etc. is here used for such things as a colour, a point in visual space etc..... "Objects" also include relations; a proposition is not two things connected by a relation. "Thing" and "relation" are on the same level. The objects hang as it were in a chain."
According to McGuinness, objects are the form of the realms of world, thought, and language. So our ‘acquaintance’ with them “is not an experience or knowledge of something over against which we stand. Thus it is not properly experience or knowledge at all [see TLP 5.552].” So it is misleading to call Wittgenstein a realist with respect to objects and,
Robert Fahrnkopf (in his Wittgenstein on Universals Peter Lang, New York, 1988) argues that Wittgenstein's objects include universals. He points out (p. 7) that Moore's notes on Wittgenstein's lectures (1930-33) report that Wittgenstein spoke of colors as if they were Russellian individuals. On p. 8 Fahrnkopf points out that in the Blue Book Wittgenstein characterizes his Tractatus view as being that redness, roundness, and sweetness are elements or individuals. He certainly seems to be talking about the universal redness rather than a particular red sense-datum here.
P. 31 of the Blue Book refers specifically to the Tractatus and the idea that a fact is a "complex of objects." The discussion there sounds like a discussion of Plato's idea of universals. Talk of facts as combinations of objects, Wittgenstein writes, springs from the following confusion: "We are misled by the substantives "object of thought" and "fact", and by the different meanings of the word "exist"."
So Wittgenstein came to think of 2.01 as a mistake. (And perhaps he already thought this way in the Tractatus, given 6.54.)
The distinction between knowledge by acquaintance and knowledge by description is, for obvious reasons, associated with Russell, but Schopenhauer makes a similar distinction in the Fourfold Root p. 163: “reason, is evidently reduced to what is possible only to abstract, discursive, reflective, and mediate knowledge that is tied to words, but not to what is possible to merely intuitive, immediate, and sensuous knowledge, which animals also share.”