It is evident that we pay careful attention to phenomena from 21 what we say about the criterion of the Sceptical School. The word criterion is used in two ways. First, it is understood as a proof of existence or non-existence, in regard to which we shall speak in the opposing argument. Secondly, when it refers to action, meaning the criterion to which we give heed in life, in doing some things and refraining from doing others, and it is about this that we shall now speak. We say, consequently, that the criterion of the Sceptical School is the phenomenon, and in calling it so, we mean the idea of it. It cannot be doubted, 22 as it is based upon susceptibility and involuntary feeling. Hence no one doubts, perhaps, that an object appears so and so, but one questions if it is as it appears. Therefore, as we cannot be entirely inactive as regards the observances of daily life, we live by giving heed to phenomena, and in an unprejudiced way. But this observance of what pertains to the 23 daily life, appears to be of four different kinds. Sometimes it is directed by the guidance of nature, sometimes by the necessity of the feelings, sometimes by the tradition of laws and of customs, and sometimes by the teaching of the arts. It is directed by the guidance of nature, for by nature we are 24 capable of sensation and thought; by the necessity of the feelings, for hunger leads us to food, and thirst to drink; by the traditions of laws and customs, for according to them we consider piety a good in daily life, and impiety an evil; by the teaching of the arts, for we are not inactive in the arts we undertake. We say all these things, however, without expressing a decided opinion. CHAPTER XII. _What is the aim of Scepticism?_