He is to give a theory of proof.That which is to be proved is a proposition;and a proposition deals with names,and moreover with the names of 'things,'not merely with the names 'of our ideas of things.'(11)That,in some sense,reasoning has to do with things is of course his essential principle;and the problem consequently arises,What are empirical 'things'?(12)Though we cannot ask what are 'ultimate things,'the logician must enumerate the various kinds of things to which reference may be made in predication.Mill makes out a classification which he proposes to substitute,provisionally at any rate,for the Aristotelian categories.(13)The first and simplest class of nameable things corresponds to things 'in the mind,'that is,'feelings,'or 'states of consciousness,'sensations,emotions,thoughts,and volitions.The second class corresponds to things 'external to the mind'.(14)and these are either 'substances'or 'attributes.'Here our task is lightened by a welcome discovery.
All philosophers,it appears,are now agreed upon one point.Sir W.Hamilton,Cousin,Kant,nay,according to Hamilton --though that is too good to be true --nearly all previous philosophers admit one truth.(15)We know,as they agree,nothing about 'objects'except the sensations which they give us and the order of those sensations.Hence the two 'substances,'body and mind,remain unknowable 'in themselves.'Body is the 'hidden external cause'to which we refer our sensations;(16)and as body is the 'mysterious something which excites the mind to feel,so mind is the mysterious something which feels and thinks.'The mind is,as he says in language quoted from his father,'a thread of consciousness,'a series of 'feelings':it is the 'myself'which is conceived as distinct from the feelings but of which I can yet know nothing except that it has the feelings.
Thus,although we know nothing of minds and of bodies 'in themselves,'we do know their existence.That is essential to his position.The 'thread of consciousness'is a 'final inexplicability'with him,but it corresponds to some real entity.And,on the other side,we must believe,in some sense,in things.The thing,though known only through the sensations which it excites,must be something more than a mere sensation,for the whole of his logic defends the thesis that in some way or other thought has to conform to facts or to the relations between 'things.'Knowledge,however,is confined entirely to the sensations and the attributes;and the two are at bottom one.The 'verbal'distinction between a property of things and the sensation which we receive from it,is made,he says,for convenience of discourse rather than from any difference in the nature of the thing denoted.(17)This brings us to a critical point.Attributes,he says,following the old distribution,are of Quality,Quantity,and Relation.Now Quality and Quantity mean simply the sensations excited by bodies.To say that snow is white,or that there is a gallon of water,means simply that certain sensations of colour or size are excited in us by snow or a volume of water.The attribute called 'Relation'introduces a different order of feelings.A 'relation'supposes that two things are involved in some one fact or series of facts.(18)But it is still an 'attribute'or a 'state of consciousness.'It is a feeling different from other feelings by the circumstance that two 'things'instead of one are involved.This is the explanation which,as we have seen,he praises so warmly in his father's Analysis,and now adopts for his own purposes.It enables him to classify predications.All predication is either an assertion of simple existence or an assertion of 'relations.'By classifying the possible relations,therefore,we obtain the possible forms of predication.It turns out accordingly that we can make five possible predications:we can predicate,first,simple existence;or secondly,'coexistence';or thirdly,'sequence'(these two being equivalent,as he adds,to 'order in place'and 'order in time');or fourthly,we may predicate 'resemblance';or fifthly,and this is only to be stated provisionally,we may predicate 'causation.'(19)So far,Mill's view corresponds to the psychology of the Analysis,which gives a similar account of the various terms employed.J.S.Mill has now the standing ground from which he can explain the whole development of knowledge.At this point,however,he has to diverge from his father's extreme nominalism.
