4.012 It is obvious that a proposition of the form ‘aRb’ strikes us as a picture.  In this case the sign is obviously a likeness of what is signified.

4.013 And if we penetrate to the essence of this pictorial character, we see that it is not impaired by apparent irregularities (such as the use [sharp] of and [flat] in musical notation).  For even these irregularities depict what they are intended to express; only they do it in a different way.

4.014 A gramophone record, the musical idea, the written notes, and the sound-waves, all stand to one another in the same internal relation of depicting that holds between language and the world.  They are all constructed according to a common logical pattern. (Like the two youths in the fairy-tale, their two horses, and their lilies.  They are all in a certain sense one.)

4.0141 There is a general rule by means of which the musician can obtain the symphony from the score, and which makes it possible to derive the symphony from the groove on the gramophone record, and, using the first rule, to derive the score again.  That is what constitutes the inner similarity between these things which seem to be constructed in such entirely different ways.  And that rule is the law of projection which projects the symphony into the language of musical notation.  It is the rule for translating this language into the language of gramophone records.

4.015 The possibility of all imagery, of all our pictorial modes of expression, is contained in the logic of depiction.

4.016 In order to understand the essential nature of a proposition, we should consider hieroglyphic script, which depicts the facts that it describes.  And alphabetic script developed out of it without losing what was essential to depiction.

4.02 We can see this from the fact that we understand the sense of a propositional sign without its having been explained to us.

4.021 A proposition is a picture of reality:  for if I understand a proposition, I know the situation that it represents.  And I understand the proposition without having had its sense explained to me.

4.022 A proposition shows its sense.  A proposition shows how things stand if it is true.  And it says that they do so stand.

4.023 A proposition must restrict reality to two alternatives:  yes or no.  In order to do that, it must describe reality completely.  A proposition is a description of a state of affairs.  Just as a description of an object describes it by giving its external properties, so a proposition describes reality by its internal properties.  A proposition constructs a world with the help of a logical scaffolding, so that one can actually see from the proposition how everything stands logically if it is true.  One can draw inferences from a false proposition.

4.024 To understand a proposition means to know what is the case if it is true. (One can understand it, therefore, without knowing whether it is true.) It is understood by anyone who understands its constituents.