How does a thought act? By being grasped and taken to be true. This is a process of the inner world of a thinker which may have further consequences in the inner world, and which may also encroach on the sphere of the will and make itself noticeable in the outer world as well. […] When a thought is grasped, it at first only brings about changes in the inner world of the one who grasps it; yet it remains untouched in the core of its essence, for the changes it undergoes affect only inessential properties. There is lacking here something we observe everywhere in physical processes—reciprocal action. Thoughts are not wholly unactual but their actuality is quite different from the actuality of things. And their action is brought about by a performance of the thinker; without this they would be inactive, at least as far as we can see. And yet the thinker does not create them but must take them as they are. They can be true without being grasped by a thinker; and they are not wholly unactual even then, at least if they could be grasped and so brought into action.

Gottlob Frege, “The Thought”, in The Frege Reader, edited by Michael Beaney