On The Vocation of Man (1799), Books I and II. What is reality?
Fichte's armchair journey starts him considering nature and thus himself as determined, but then he backtracks to say that actually, experience doesn't tell us whether we're determined or free. In Book II, he argues that since our experience is always of something going on in ourselves, then causality, the external world, the self, etc. must be our own mental creations. So we're free after all, yet everything is drained of significance!
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The Partially Examined Life Philosophy Podcast