Tractatus Logico-Philosophicus by Ludwig Wittgenstein + comments  by Maarten Maartensz   

4 A thought is a proposition with a sense.

I doubt very much this is a helpful way of speaking about thoughts or propositions. Let me first note a number of points incidental to the formulation, and then raise two more general objections.

I have a lot of mental imagery that is not propositional, but is what I would call thinking, and the same holds for most people. Incidentally, the term 'proposition' as used in English Analytical Philosophy is very vague and ambiguous. Mostly, its meaning varies between that of 'statement' and 'idea'. Here I will take it that W. means or tends to mean 'statement' rather than 'idea'.

That many thoughts may be given a linguistic expression, and then are stated by propositions with a sense (or meaning) is an interesting fact, but should not induce people to confuse thoughts with their linguistic expressions. (The difference is intuitively clear if you know more than one language.)

Also, it would be nice to allow for the possibility that other animals may be thinking, and without propositions to do it with.

This last remark is related to the more more general objections:

Of course there is thought without propositions, and even if the capacity to think were denied to orang utans, bonobos, chimps, rooks and foxes, still human babies do have to learn to speak, and presumably do think while learning to speak.

And as I started with saying, at least in my case, and that of quite a few others, judged by William James' Principles of Psychology, there is rather a lot more to thinking than verbal thinking, and indeed verbal thinking would be much more difficult to explain if we did not assume that it represents non-verbal thinking, even if this is not conscious.

So... in [4] we are told something that is misleading, since it suggests there are no thoughts that are not linguisticn ('propositions'), somehow, and unclear, for in fact 'thought', 'proposition' and 'sense' are all quite unclear.