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.'22We 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.'23The 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,'24Whatever,then,may be the origin of the belief --'intuition'or 'association'--it is clear that it can give us no knowledge except such as is derived from sensations.
Moreover,Brown is thus led,as in the doctrine of causation,to accept a really sceptical position.He declares that he is in this respect at one with both Reid and Hume.They both accept two propositions:first,that we cannot 'by mere reasoning'prove the existence of an external world;secondly,that it is 'absolutely impossible for us not to believe'in its existence.Hume,he says,pronounces the first proposition in a 'loud tone of voice'and 'whispers'the second.Reid,conversely,passes over the first rapidly and 'dwells on the second with a tone of confidence.'25Brown accepts both statements.He has already said that there is no argument against Berkeley's denial of matter any more than against the 'infinite divisibility of matter.'But he adds,it is 'physically impossible'for us to admit the conclusion,at least without 'an instant dissent from a momentary logical admission.'26This,indeed,is but a version of Hume's familiar statement that Berkeley's arguments admit of no reply and produce no conviction.
Another essential doctrine of the Mills,the 'association'theory,is treated differently by Brown.
Brown,as we have seen,both in his theory of causation and in his theory of our belief in an external world,speaks of principles in the mind which somehow override 'ratiocination,'in the first case,he speaks of 'intuition,'but in the other,as I have said,he seems to prefer association,the difference is remarkable because the belief in an external world is upon his showing simply a case of causation.It means essentially the reference of our sensations as to an external cause.Now,in the argument upon causation,he has insisted upon the insufficiency of association to generate the belief;and he would have found it difficult to meet his own arguments if applied to the belief in an external world.Yet it does not seem to occur to him that there is any difficulty in explaining this belief in an external world as a case of what Mill called 'indissoluble association.'Brown,as Mill thought,was not sufficiently aware of the power of this principle,and the difference between them is marked by this divergence.Brown had a great deal to say about association,though he chose generally to substitute the word 'suggestion,'previously familiar to Reid and Berkeley.27He considers it,however,mainly in another relation.He proposes to trace the order in which 'trains'of ideas succeed each other in our minds.He does not dwell upon the influence of association in producing belief.His question is not primarily as to the logic,but as to the actual succession of our thoughts.