Descartes, for example, makes the distinction between body and mind by recognizing the body as an idea born from 'extension' and the mind as an idea born from 'consciousness'. He further recognizes 'extension' as that which is made up of geometric properties, and 'consciousness' as that made up of thought -- both of which (presumably) existed before we did, and thus we were born to them.
I am curious how we were able to make these initial distinctions, however. Because in understanding what the body is, we must first understand the body as a property of extension, and thus distinguished from the property of consciousness. And then to do this, we must also recognize the properties of geometry, and be able to distinguish them from the properties of thought. Presumably, this would keep going: we would need to distinguish between shapes and features of those shapes in order to understand what geometry encompasses, for example.
I am wondering if there exists a philosophical answer (or attempted answer) to understand what came before our very first distinction. What then led us to inevitably make that first distinction? Perhaps the 'tertiary qualities' that Descartes discusses (pleasure, pain, etc.) motivated us towards or away from certain objects, thus inevitably creating distinctions in our minds?