Let me repeat in slightly altered terms some of the points raised in the preceding sections.  I have doubted and denied that there are identically similar objective experiences; I consider all objective beings as individual and unique.  It is now understood that conceivably only in the subjective world, and in theory and the imagination, do we deal with identically similar units, and with absolutely commensurable quantities.  In the real world it is reasonable to suppose we deal at most with practically similar units and practically commensurable quantities.  But there is a strong bias, a sort of labour-saving bias, in the normal human mind, to ignore this, and not only to speak but to think of a thousand bricks or a thousand sheep or a thousand Chinamen as though they were all absolutely true to sample.  If it is brought before a thinker for a moment that in any special case this is not so, he slips back to the old attitude as soon as his attention is withdrawn.  This type of error has, for instance, caught many of the race of chemists, and atoms and ions and so forth of the same species are tacitly assumed to be similar to one another.

Be it noted that, so far as the practical results of chemistry and physics go, it scarcely matters which assumption we adopt, the number of units is so great, the individual difference so drowned and lost.  For purposes of enquiry and discussion the incorrect one is infinitely more convenient.

But this ceases to be true directly we emerge from the region of chemistry and physics.  In the biological sciences of the eighteenth century, common-sense struggled hard to ignore individuality in shells and plants and animals.  There was an attempt to eliminate the more conspicuous departures as abnormalities, as sports, nature’s weak moments; and it was only with the establishment of Darwin’s great generalizations that the hard and fast classificatory system broke down and individuality came to its own.  Yet there had always been a clearly felt difference between the conclusions of the biological sciences and those dealing with lifeless substance, in the relative vagueness, the insubordinate looseness and inaccuracy of the former.  The naturalist accumulated facts and multiplied names, but he did not go triumphantly from generalization to generalization after the fashion of the chemist or physicist.  It is easy to see, therefore, how it came about that the inorganic sciences were regarded as the true scientific bed-rock.  It was scarcely suspected that the biological sciences might perhaps after all be truer than the experimental, in spite of the difference in practical value in favour of the latter.  It was, and is by the great majority of people to this day, supposed to be the latter that are invincibly true; and the former are regarded as a more complex set of problems merely, with obliquities and refractions that presently will be explained away.  Comte and Herbert Spencer certainly seem to me to have taken that much for granted.  Herbert Spencer no doubt talked of the unknown and unknowable, but not in this sense as an element of inexactness running through all things.  He thought, it seems to me, of the unknown as the indefinable Beyond of an immediate world that might be quite clearly and definitely known.