What is the difference between philosophical idealism, such as the idealism of Kant, and the meaning generally given to being an "idealist?"

It's the difference between thinking that everything is, ultimately, made out of ideas (what we think of as the physical world is somehow a mental construction) and having ideals (and optimistically thinking that people can and should live up to them).

It is perhaps worth pointing out, belatedly, that Kant's idealism (in the first sense) also includes idealism (in the second sense). For example, the ideas of pure reason (the topic of the 'Dialectic' chapters of the Critique of Pure Reason) have a role in our thoughts concerning the nature of reality. This is idealism in the first sense. However, that role functions by being something akin to an 'ideal' in the second sense. Specifically, the ideas of pure reason function 'regulatively', by guiding our thought towards something that is strictly speaking impossible, but the being guided is never-the-less important for us.

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