"It is the presence of life, we say, which makes the difference between that which has soul and that which has not. To amplify regarding life: we call anything alive which possesses even a single one of the following: intellect, sensation, motion and rest in space, and also the motion[274] involved in nutrition, and both decay and growth. Therefore, even all the plants are held to be alive." A few lines further on Aristotle says, speaking of the power or faculty[275] of taking nourishment:-- "This can exist without the others, but not the other faculties without this, in mortal beings. The aforesaid is clear in the case of plants; for they possess no other faculty of the soul. To this faculty then life owes its origin in living things; but the being an animal owes its origin primarily to sensation; for beings that neither move nor change their place but yet possess sensation, we call animals and not merely living things. The primary sense, which exists in all, is touch; and just as the nutritive faculty can exist without touch or sensation of any kind, so can touch exist without the other senses. The "nutritive" is our term for such part of the soul as is shared even by plants, all animals, however, evidently possessing the sense of touch. The cause of the presence of each of the two aforesaid shall be told later. Now let us only go so far as to say that the soul is the source of the [faculties] aforesaid, and is defined by means of them, to wit: the nutritive, the sensory, the intellectual, the motor.[276] As to whether each of these is a soul or is a part of the soul; and if a part, whether in the sense that it is only separable by reasoning,[277] or locally as well--as to some of these points, it is not hard to see our way, but some present difficulties."[278] If we turn to Aristotle's treatise On Generation we find him dealing with the relations of the body to the nutritive soul, in virtue whereof the body is alive; with its relations to the sensory soul, in virtue whereof it is an animal body; and, finally, in man with its relations to the intellectual soul. Of these three kinds of soul or parts of the soul, he concludes, the mind "is alone divine; for in the working thereof no bodily working is involved."[279] Only soul of this divine quality does he admit to be separable from body.[280] The master has spoken. Now let the great pupil speak. In the last Exercise but one of his treatise On Generation, Harvey says, referring to the blood:--