§§40--42. Summary. The Academics have a regular method. They first give a general definition of sensation, and then lay down the different classes of sensations. Then they put forward their two strong arguments, (1) _things_ which produce _sensations_ such as might have been produced in the same form by other _things_, cannot be partly capable of being perceived, partly not capable, (2) _sensations_ must be assumed to be of the same form if our faculties do not enable us to distinguish between them. Then they proceed. Sensations are partly true, partly false, the false cannot of course be real _perceptions_, while the true are always of a form which the false _may_ assume. Now sensations which are indistinguishable from false cannot be partly perceptions, partly not. There is therefore no sensation which is also a perception (40). Two admissions, they say, are universally made, (1) false sensations cannot be perceptions, (2) sensations which are indistinguishable from false, cannot be partly perceptions, partly not. The following two assertions they strive to prove, (1) sensations are partly true, partly false, (2) every sensation which proceeds from a reality, has a form which it might have if it proceeded from an unreality (41). To prove these propositions, they divide perceptions into those which are sensations, and those which are deduced from sensations; after which they show that credit cannot be given to either class (42). [The word "perception" is used to mean "a certainly known sensation."]