Geulincx principally deals with the question, left in an obscure and unsatisfactory state by Descartes, of the relation between soul and body. Whereas Descartes made the union between them a violent collocation, Geulincx practically called it a miracle. Extension and thought, the essences of corporeal and spiritual natures, are absolutely distinct, and cannot act upon one another. External facts are not the causes of mental states, nor are mental states the causes of physical facts. So far as the physical universe is concerned, we are merely spectators; the only action that remains for us is contemplation. The influence we seem to exercise over bodies by will is only apparent; volition and action only accompany one another. Since true activity consists in knowing what one does and how one does it, I cannot be the author of any state of which I am unconscious; I am not conscious of the mechanism by which bodily motion is produced, hence I am not the author of bodily motion ("Quod nescis quomodo fiat, id non facis"). Body and mind are like two clocks which act together, because both have been set together by God. A physical occurrence is but the occasion (opportunity, occasional cause) on which God excites in me a corresponding mental state; the exercise of my will is the occasion on which God moves my body. Every operation in which mind and matter are both concerned is an effect of neither, but the direct act of God. Geulincx was thus the first definitely to systematize the theory called Occasionalism, which had already been propounded by Gérauld de Cordemoy (d. 1684), a Parisian lawyer, and Louis de la Forge, a physician of Saumur. But the principles on which the theory was founded compelled a further advance. God, who is the cause of the concomitance of bodily and mental facts, is in truth the sole cause in the universe. No fact contains in itself the ground of any other; the existence of the facts is due to God, their sequence and coexistence are also due to him. He is the ground of all that is. My desires, volitions and thoughts are thus the desires, volitions and thoughts of God. Apart from God, the finite being has no reality, and we only have the idea of it from God. Descartes had left untouched, or nearly so, the difficult problem of the relation between the universal element or thought and the particular desires or inclinations. All these are regarded by Geulincx as modes of the divine thought and action, and accordingly the end of human endeavour is the end of the divine will or the realization of reason. The love of right reason is the supreme virtue, whence flow the cardinal virtues, diligence, obedience, justice and humility. Since it is impossible for us to make any alteration in the world of matter, all we can do is to submit. Chief of the cardinal virtues is humility, a confession of our own helplessness and submission to God. Geulincx's idea of life is "a resigned optimism."