Brown has reversed the interpretation of Reid's _experimentum crucis_. I will give up my case, says Reid, if you can make the external world out of sensations. That, replies Brown, is precisely what we can do. How from sensations do we get what Berkeley called 'outness'? We get it, says Brown, from the sense of resistance or 'impeded effort.' That reveals to us the fact that there is something independent of ourselves, and the belief in such a something is precisely what we mean, and all that we mean, by the belief in an external world. Consistently with this, Brown rejects Reid's distinction between the primary and secondary qualities. The distinction corresponds no doubt to some real differences, but there is no difference of the kind suggested by Reid. 'All [the qualities] are relative and equally relative--our perception of extension and resistance as much as our perception of fragrance and bitterness.'[485] We ascribe the sensations to 'external objects,' but the objects are only known by the 'medium' of our sensations. In other words, the whole world may be regarded as a set of sensations, whether of sight, smell, touch, or resistance to muscular movement, accompanied by the belief that they are caused by something not ourselves, and of which something we can only say that it is not ourselves. Once more, the analysis of the process by which the belief is generated is significant. From resistance, or the sensation produced when something 'resists our attempts to grasp it,' we get the 'outness.' Then perception is 'nothing more than the association of this complex notion with our other sensations--the notion of something extended and resisting, suggested by these sensations, when the suggestions themselves have previously arisen, and suggested in the same manner and on the same principle as any other associate feeling suggests any other associate feeling.'[486] The odour or colour of a rose recalls the sensation of touching and of resistance to our grasp. Thus we regard the whole group of sensations as due to the external cause which produces the sensation of resistance. Brown seems to hesitate a little as to whether he shall appeal to an 'intuition' or to 'association,' but 'as I rather think,' he says, the belief is founded 'on associations as powerful as intuition.'[487]