Edmund Husserl has a beef with Descartes. In Cartesian Meditations, sec. 10, Husserl alleges that the Frenchman fails to make the transcendental turn (die transzendentale Wendung). He stops short at a little tag-end of the world (ein kleines Endchen der Welt), from which he then argues to get back what he had earlier doubted, including the external world. Despite his radical doubt, Cartesius remains within the world thinking he has found the sole unquestionable part of it.
Descartes replaces ego with substantia cogitans, mens sive animus. This give rise to what Husserl calls the absurdity of transcendental realism. Husserl's thought seems to be that if one fully executes the transcendental turn one is left with no entity existing in itself on which one can base anything. Everything objective acquires its entire Seinsgeltung (ontic validity) from the transcendental ego, including any thinking substances there are.
Now I'm perplexed. Just what is this transcendental ego if it is the purely subjective source of all Seinsgeltung? Is it at all? If it is or exists at all, then it is in the world, even if not in the physical world. It is in the world as the totality of entities. But it can't be inasmuch as the transcendental ego as the constitutive source of all ontic validity is pre-mundane.
The puzzle could be put like this. Either the constitutive source of all Seinsgeltung is pre-mundane or it is not. If the former, then it would appear to be nothing at all. If the latter, then it is not the constitutive source of all Seinsgeltung.