Like the Epicureans, Zeno and his successors made philosophy primarily a search for the chief good, a doctrine of practice and morals. But like them they were impelled to admit a logic and a physics, at least by way of preliminary basis to their [390] ethics. The relations of the three they illustrated by various images. Philosophy was like an animal; logic was its bones and sinews, ethics its flesh, physics its life or soul. Or again, philosophy was {230} an egg; logic was the shell, ethics the white, physics, the yolk. Or again, it was a fruitful field; logic was the hedge, ethics the crop, physics the soil. Or it was a city, well ordered and strongly fortified, and so on. The images seem somewhat confused, but the general idea is clear enough. Morality was the essential, the living body, of philosophy; physics supplied its raw material, or the conditions under which a moral life could be lived; logic secured that we should use that material rightly and wisely for the end desired. [391] Logic the Stoics divided into two parts--Rhetoric, the 'science of the open hand,' and Dialectic, the 'science of the closed fist,' as Zeno called them. They indulged in elaborate divisions and subdivisions of each, with which we need not meddle. The only points of interest to us are contained in their analysis [392] of the processes of perception and thought. A sensation, Zeno taught, was the result of an external _impulse_, which when combined with an internal _assent_, produced a mental state that revealed at the same time itself and the external object producing it. The perception thus produced he compared to the grip which the hand took of a solid object; and real perceptions, those, that is, which were caused by a real external object, and not by some illusion, always testified to the reality of their cause by this sensation of 'grip.' {231} The internal assent of the mind was voluntary, and at the same time necessary; for the mind could not do otherwise than will the acceptance of that which it was fitted to receive. The peculiarity of their physics, which we shall have to refer to later on, namely, the denial of the existence of anything not material, implied that in some way there was a material action of the external object on the structure of the perceiving mind (itself also material). What exactly the nature of this action was the Stoics themselves were not quite agreed. The idea of an 'impression' such as a seal makes upon wax was a tempting one, but they had difficulty in comprehending how there could be a multitude of different impressions on the same spot without effacing each other. Some therefore preferred the vaguer and safer expression, 'modification'; had they possessed our modern science, they might have illustrated their meaning by reference to the phenomena of magnetism or electricity.