capable of definition and demonstration.] Ph. The simple ideas which come by reflection are the ideas of the understanding and of tie will [for we ourselves perceive them in reflecting upon ourselves.] . Th. [It is doubtful if all these ideas are simple, for it is clear, for example, that the idea of the will includes that of the understanding, and that the idea of motion contains that of figure. . Ph. There are some simple ideas which make themselves perceived in the mind by all the avenues of sensation and by reflection also — viz.:' pleasure, pain, power, existence, unity. Th. [It seems that the senses cannot convince us of the existence of sensible things without the aid of the reason. Thus I should think that the idea of existence comes from reflection. That of power also and of unity come from the same source, and are of a wholly different nature from the perceptions of pleasure and pain.] . Ph. What shall we say of ideas of privative qualities? It seems to me that the ideas of rest, darkness, and cold are as positive as those of motion, light, and heat. Nevertheless, in proposing these privations as the causes of privative ideas I follow the common view; but in the main it will be difficult to determine whether there is really any idea which arises from a privative cause until it has been determined whether rest any more than motion is a privation. Th. [I had not believed that we could have reason to doubt the privative nature of rest. It suffices it that motion in the body be denied, but it does not suffice for motion to deny rest, and it is necessary to add something more to determine the degree of motion, since it receives materially more or less, while all rest is equal. It is another thing when we speak of the cause of rest, which must be positive in the secondary' matter or mass. I should furthermore regard the very idea of rest as privative — i.e., that it consists only in negation. It is true that the act of denial is positive.) . Ph. The qualities of things being the faculties they have of producing in us perception of ideas, it is well to distinguish these qualities. They are primary and secondary. Extension, solidity, figure, number, mobility are the original qualities inseparable from body which I call primary. . But I call secondary qualities the faculties or powers of bodies to produce certain sensations in us, or certain effects in other bodies, as the fire, for example,