Which is the exact antecedent of ‘that’?

What is the meaning of ‘medium’ in this context?

The passage below is from ‘the Blank Slate’ by Steven Pinker.

http://evolbiol.ru/blankslate/blankslate.htm
Evolutionary psychology also explains why the slate is not blank. The mind was forged in Darwinian competition, and an inert medium would have been outperformed by rivals outfitted with high technology — with acute perceptual systems, savvy problem-solvers, cunning strategists, and sensitive feedback circuits. Worse still, if our minds were truly malleable they would be easily manipulated by our rivals, who could mold or condition us into serving their needs rather than our own. A malleable mind would quickly be selected out.

In this passage I cannot pinpoint the exact meaning of the underlined ‘medium’ in this context. It seems to refer to ‘mind’ but I’m not sure.

Can you show me what it represents?
(And since the lexical meaning of ‘medium’ is actually ‘something intermediate in nature or degree,’ or ‘an intervening substance’ how can it possibly refer to ‘mind,’ or whatever it is?)

Hope for your replies.
Regards.