system
Given the following post, respond as if you're commenting on Reddit:
human
[philosophy of math] I don't understand a specific argument made by Øystein Linnebo and Frege in favor of realism and as a challenge for intuitionism/constructivism regarding mathematical truths before discovery

I'm having trouble with an argument in Øystein Linnebo's book Philosophy of Mathematics. 

As a challenge for people who hold the position that mathematical objects are mental constructions he advances the following argument:
> Some related problems would still remain, however. By making mathematics dependent on human thought, even if not on
individual acts thereof, we would still ascribe incorrect temporal  and modal properties to mathematical truths. As Frege famously
argued, all mathematical truths now known were true already
before we discovered them, indeed even before the existence of
the human species (cf. §1.3). This is why it is permissible to
apply mathematics, as we now have it, to theorize about not
only the present and future, but also the distant past. It is hard
to see how claims that were true even before the arrival of the
human species can be said to be “dependent on human thought.”

Especially in the case of constructivists, I don't see that at all.
To me that seems like he's applying presupposed notions of existence and truth (those of realism) to a position that holds entirely different notions of existence and truth, without adjusting anything.

One could simply respond: Yes, mathematicians *could* have constructed the same objects and truths before the person who became famous for doing so did it. And it *would* have had the same implications. But nobody *did*, therefore it wasn't a mathematical truth, there was no such thing.  The fact that we share our intuition with a lot of people and that other people could have made the same mental construction earlier doesn't imply that those constructions exist independent from our mind.

I guess I understand where he's coming from but without previously subscribing to some sort of realism, that argument just seems much weaker to me and imply far less than he apparently thinks it does. To take it as a strong argument for realism just seems to beg the question of mathematical truths and objects.
gpt
Your response seems good to me. 

In general you should be aware that realists often have trouble putting on their "anti-realist hats" and thus seeing things from that perspective. After all: the realist position necessarily logically conflates things that the anti-realist does not. So for people to misunderstand the position they're arguing against isn't a huge surprise.

Also while Linnebo seems to be arguing here against constructivists that is *not* Frege's opponent, because those positions mostly post-date him. Frege is arguing against psychologism, and although one can give a psychologistic constructivism, they needn't.