Every given reality implies the thought which apprehends it. Therefore being is the condition of knowing; knowing is not the condition of being. - Étienne Gilson

The mathematician always proceeds from thought to being or things. Consequently, critical idealism was born the day Descartes decided that the mathematical method must henceforth be the method for metaphysics.

Indeed, all idealism derives from Descartes, or from Kant, or from both together, and whatever other distinguishing features a system may have, it is idealist to the extent that, either in itself, or as far as we are concerned, it makes knowing the condition of being.

The realist method starts with the whole in order to distinguish the parts.