In War Diaries, Sartre discusses 'appropriation'. Firstly, his lack of desire to own objects, to appropriate things materially. He is indifferent to the usual object-expressions of the personality. He sees the desire to own things as a desire to represent yourself, to embody yourself through stand-ins and symbols. He is self-sufficient, and so does not need such embodiments.
But then he goes on to say that he appropriates the world not materially but by thinking, by extracting its meaning. Hence, he appropriates its essence. But he must then transmute it into a second material form, the sentence. The sentence for Sartre is a kind of monad: it captures or reflects the world but remains at the same time a self-sufficient object:
This possession consists.. in capturing the world’s meaning by sentences [..] the sentence which captures satisfies me only if it itself an object; in other words, if the meaning of the world appears in it, not in its conceptual nakedness but via a material. The meaning must be captured with the aid of a capturing thing, which is the aesthetic sentence: an object created by me and existing by itself alone.
In childhood, as described in Words there is a slightly different sense of the transmutation of thought into matter.
Nothing disturbed me more than to see my scrawls little by little change their will-o'-the-wisp gleam for the dull consistency of matter: it was the realization of the imaginary. Caught in the trap of naming, a lion, a captain of the Second Empire, or a Bedouin would be brought into the dining room; they remained captive there forever, embodied in signs.I thought I had anchored my dreams in the world by the scratchings of a steel nib.