Since the other comment was a bit flippant and didn’t really understand or care to explain genuinely.
I believe Materialism here is being used as a subset of a broader Marxist idea of Dialectical Materialism. The whole concept is pretty dense, but the core idea, and the subset being used here, is that the conditions people live in are the larger driver of their actions. So crime does not come from an innate spiritual poison one possesses, but from conditions that inspire criminal behavior in people, such as poverty and wealth inequality.
“Material conditions” is a common leftist phrase that summarizes the ideas materialism, so speaking to ‘conditions’ as Mamdani did can be a bit of a dog whistle for leftist ideas.
In short, to quote Kamala Harris, “You think you just fell out of a coconut tree? You exist in the context of all in which you live and what came before you”
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