Mach does not think, of course, that there is nothing outside our mind. On the contrary, he is interested precisely in what is outside our minds (whatever the ‘mind’ is): nature, in all its complexity, of which we are a part. Nature presents itself as a set of phenomena, and Mach recommends the study of those phenomena in order to build syntheses and conceptual structures that make sense of them, rather than to postulate a priori underlying realities.
