I have seen the argument of surjecting a countable set of spatio-temporal coordinates on the set of all ideas including all real numbers that ever have been or will be realized: Countability of the real numbers, p. 275 of https://www.hs-augsburg.de/~mueckenh/Transfinity/Transfinity/pdf. Since no reference to earlier work is given, I assume this is the first appearance of the idea unless earlier authors can be identified.
EDIT(1): In the writings of Borel, Lebesgue, Peirce, Russell, or Moore's "Zermelo's Axiom of Choice" I have not been able to find this idea. Hadamard was anyway in favour of Zermelo's approach to set theory.
Edit(2): Here is a nice definition of thought-object:
At request of the referee who asked what is a thought-object let me add: I understand it to be a thought about an object which may exist or not. Thus it is an electrochemical event in the brain or/and its record in the memory. In particular it is a physical thing in space time. Of course it is difficult to characterise any physical phenomena. But we have the ability to recognize thoughts as identical or different, just as we have the ability to recognize a silent lightning from a thunderous one. Hence I understand Hilbert's words as follows: mathematicians imagine sets which do not exist, but their thoughts about sets do exist and they can arise prior to the thoughts of most elements in those sets. (J. Mycielski: "Russell's paradox and Hilbert's (much forgotten) view of set theory" in G. Link (ed.): "One hundred years of Russell's paradox: mathematics, logic, philosophy", de Gruyter, Berlin (2004) p. 534)