3.04 An a priori true thought would be one whose possibility guarenteed its truth.

Note: I am asking about the earlier Wittgenstein and not his post-Tractatus philosophy.

It is clear to me why this is not an a priori true fact, as the act of thinking would be an aspect of the world being appealed to. But I don't understand why it is not an a priori true thought.

  • "true thought" as in it is an actual thought (proposition with a sense per TLP) or a thought that's propositional content is true? Welcome to philosophy.SE!
    – MmmHmm
    Commented Mar 15, 2017 at 17:39
  • 3.04 sounds strikingly like the ontological argument for existence, stripped of it's religious nature. Commented Mar 15, 2017 at 17:42
  • In Tractatus a thought is not an "act of the mind", but a "thought content". Compare with G.Frege, The Thought: A Logical Inquiry (1918). Commented Mar 15, 2017 at 18:59
  • "a priori true fact" is not W... True are thought and propositions that "picture" the facts. Commented Mar 15, 2017 at 19:24
  • See also Juliette Floyd, The Frege-Wittgenstein Correspondence for the relation between W's Tractatus and later Frege, specifically The Thought. Commented Mar 15, 2017 at 19:57

2 Answers 2


The thing is, that for the early Wittgenstein the Cogito Ergo Sum was just not true. So the Cogito could not be true a priori for him.

Like David Hume, Wittgenstein believed that the Cartesian Ego, the thinking subject, was nowhere to be found.

5.631 There is no such thing as the subject that thinks or entertains ideas. If I wrote a book called The World as l found it , I should have to include a report on my body, and should have to say which parts were subordinate to my will, and which were not, etc., this being a method of isolating the subject, or rather of showing that in an important sense there is no subject; for it alone could not be mentioned in that book.

The self exists only as an external "limit" of the linguistically constructed world. (Perhaps akin to Kant's Transcendental Subject)

5.632 The subject does not belong to the world: rather, it is a limit of the world.

And again

5.641... The philosophical self is not the human being, not the human body, or the human soul, with which psychology deals, but rather the metaphysical subject, the limit of the world — not a part of it.


It seems to me that there is no place for a priori knowledge in the Tractatus (quoting from Pears/McGuinness translation).

Some basic concepts are : form, possibility, picture.

3 A logical picture of facts is a thought.

Here I read "thought" in a Fregean way, as "thought content" (and not as an act of the mind).

A picture can represent reality in virtue of its (logical or pictorial) form (2.17).

2.11 A picture presents a situation in logical space, the existence and non-existence of states of affairs [see also 2.202].

Thus, we have thoughts that "represent" possible situations [their sense; see 2.221] and true thoughts that represent "actual" situations: facts (2.222 and 3.01).

2.225 There are no pictures that are true a priori.

Now for

3.04 If a thought were correct a priori, it would be a thought whose possibility ensured its truth.

This is reminiscent of the well-known ontological arguments: a concept (or thought) that "analytically" entails the existence of an object that falls under the concept; whose possibility implies its actuality.

3.05 A priori knowledge that a thought was true would be possible only if its truth were recognizable from the thought itself (without anything to compare it with).

Thus, it seems that there are no a priori truth.

5.133 All deductions are made a priori.

This fits with the "tautological" nature of logic: logic "transform" but not "produce" truth and knowledge [see also: 5.4731 What makes logic a priori is the impossibility of illogical thought.]


5.134 One elementary proposition cannot be deduced from another.

I.e. there is no knowledge by "logic alone" of the contingent facts of the world. See also :

5.557 The application of logic decides what elementary propositions there are. [...]

5.5571 If I cannot say a priori what elementary propositions there are, then the attempt to do so must lead to obvious nonsense.

Now for the cogito: in Descartes it is not a deduction but an intuition. If so, it must be modelled in Tractarian's philosophy as an elementary proposition, that cannot be deduced from another.

If it is true, it is so by way of its picturing a fact.

But a picture is always contingent: there are no a priori pictures.

Thus, it can be (at most) the "empirical" recognition of a contingent fact.

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