When he comes to modes, he examines them by the doctrine {137} of abstract or general ideas propounded by Berkeley, which he characterizes "as one of the greatest and most valuable discoveries that has been made of late years in the republic of letters." According to this very defective theory (as it appears to us), all abstract or general ideas are nothing but particular ones annexed to a certain term.Like Locke, Hume confounds abstract and general ideas, which should be carefully distinguished: the former meaning the notion of the part of an object as a part, more particularly an attribute; the other, the notion of objects possessing common attributes, the notion being such that it embraces all the objects possessing the common attributes.Abstraction and generalization are most important intellectual operations, the one bringing specially to view what is involved in the concrete knowledge (not impression) of the individual, and the other exhibiting the qualities in respect of which objects agree.Without such elaborative processes, we should never know all that is involved in our original perceptions by sense and consciousness.Nor is it to be forgotten, that when the concrete is a real object, the abstract is a real quality existing in the object; and that when the singulars are real, the universal is also real, that is, a class all the objects in which possess common qualities.Here again we find Hume overlooking one of the most essential of our mental attributes, and thus degrading human intelligence.In relation to the particular end for which he introduces his doctrine, I hold that substance and mode are known in one concrete act and that we can separate them by abstraction for more particular consideration; the one having quite as real an existence as the other, and both having their reality in the singular object known by sense and consciousness.
He goes on to a very subtle discussion as to our ideas of space and time.He says, that "it is from the disposition of visible and tangible objects we receive the idea of space, and from the succession of ideas and impressions we form the idea of time." The statement requires to be amended.It is not from the disposition of separate objects we have the idea of space, but in the very perception of material objects we know them as extended, that is, occupying space; and in the very remembrance of events we have time in the concrete, that is, events happening in time past.He is therefore wrong in the {138} sceptical conclusion which he draws, that the ideas of space and time are no distinct ideas; for they are ideas formed by a high intellectual process from things immediately known.Taking a defective view of the nature and function of abstraction, he denies that we can form any idea of a vacuum or extension without matter.He maintains that the idea we form of any finite quality is not infinitely divisible.The dispute, he says, should not be about the nature of mathematical points, but about our ideas of them; and that, in the division of our ideas, we come to a minimum, to an indivisible idea.
This whole controversy seems to me to arise from a misapprehension.Our idea of space, it is evident, is neither divisible nor indivisible and as to space, it is not divisible either finitely or infinitely for while we can divide matter, that is, have a space between, we cannot separate any portion of space from all other space: space is and must be continuous.He is evidently jealous of the alleged certainty of mathematics, which seemed to be opposed to his universal scepticism.He maintains that the objects of geometry are mere ideas in the mind.I admit that surfaces, lines, points, have no independent existence, but they have all an existence in solid bodies.By an excess of ingenuities and subtleties, he would drive us to the conclusion that space and time are mere ideas, for which we need not seek a corresponding reality; a conclusion unfortunately accepted by Kant, who thus opened the way to the empty idealism which so long reigned in the German philosophy. {139}
The result reached is summed up in the statement: "As long as we confine our speculations to the <appearances> of objects to our senses, without entering into disquisitions concerning their real nature and operations, we are safe from all difficulties, and can never be embarrassed by any question; " but, " if we carry our inquiry beyond the appearances of objects to the senses, I am afraid that most of our conclusions will be full of scepticism and uncertainty." The intelligent reader will here perceive the source whence Kant derived his doctrine that the senses give us, not things, but <phenomena>, that is appearances, and that we are involved in contradiction when we suppose that they furnish more.However great the logical power of the German metaphysician, it is clear that he did not possess the shrewdness of the common-sense philosopher of Scotland, when he adopted the conclusion of the sceptic as his starting-point.
He has now to face the important subjects of existence and knowledge.Proceeding on his assumption that nothing is present to the mind but perceptions, he argues, I think logically (if the premises be allowed), that we can never advance a step beyond ourselves, and that it is " impossible for us so much as to conceive or form an idea of any thing specifically different from ideas or impressions." As knowledge had been represented by Locke as consisting in comparison (I reckon this a false and dangerous doctrine), Hume has to consider the relations which the mind of man can discover.