For all the modes of thinking of which we are conscious may be referred to two general classes, the one of which is the perception or operation of the understanding, and the other the volition or operation of the will. Thus, to perceive by the senses (sentire), to imagine, and to conceive things purely intelligible, are only different modes of perceiving (percipiendi); but to desire, to be averse from, to affirm, to deny, to doubt, are different modes of willing. + 