The Reasons for the Comparison

Although Spinoza breaks completely with Descartes in order to found a new science of the affects in the Preface to Ethics III, the fact remains that the author of the Passions of the Soul left a mark on the former's youthful works. Indeed, in the Short Treatise, the analysis of the nature of the affects still embraces a Cartesian model on at least two levels.

First, the nature of the causality that governs the relationship between the soul and the body remains in part Cartesian. Admittedly, Spinoza distances himself from Descartes by refusing to accept that the body is the principal cause of the passions of the soul. The proximate cause of the affects (lijdingen) in the soul is knowledge – in other words, a mode of thinking. Affects can only be experienced by a being who conceives and who is constituted by modes of thinking. Thus love, desire, and all of the affects in general stem from three different modes of knowledge by which humans understand themselves: opinion (waan), which is based on a conception through hearsay and experience; true belief (geloof), which involves apprehending things and their necessity through reason without direct vision; and clear knowledge (klaare kennisse), which does not come from being convinced by reasons but from the awareness and enjoyment of the thing itself. All affects and passions, such as hate and aversion, that are contrary to right reason come from opinion. All affects or passions that are good desires, such as legitimate self-satisfaction and humility, which encompass knowing one's perfection according to its true worth, arise from belief. Finally, clear knowledge produces pure, true love, namely the love of God.

Although all affects are the product of a mode of knowledge, Spinoza nevertheless continues to accept the idea of reciprocal action of the soul on the body and vice versa. In the Short Treatise, Spinoza is therefore partly subject to the very critiques he would later elaborate in Part III of the Ethics, since he contemplates the possibility of an interaction between the soul and the body and one producing an effect on the other.