But here we must return to the argument, already mentioned in passing, which may be restated thus: A thousand labourers, directed by their own intelligence only, produce a product whose amount we will call a thousand. The same labourers are directed by a man of ability, and the product rises from one thousand to two. But if the production of this second thousand is to be credited to the man of ability on the ground that, were the ability absent, no second thousand would be produced, we may reach by the same reasoning a conclusion precisely opposite, and credit not only the first, but both the thousands to labour, on the ground that, if the labour were absent, nothing would be produced at all. The argument is plausible; and in order to understand its fallacy we must give our attention to a fact, not generally realised, which is involved in all practical reasoning about all causes whatsoever. If we use the word "cause" in its strict speculative sense, the number of causes involved in the simplest effect is infinite. Let us take, for example, the speed of a horse which wins a race. Why does the speed of this horse exceed that of the others? We may in answer point to qualities of its individual organism. But these will carry us back to all its recorded ancestors--sires and dams for a large number of generations: and even so we shall have been taken but a small part of our way. The remotest of these ancestors--why were they horses at all? For our answer we must travel through the stages of organic evolution, till we reach the point at which animal and vegetable life were one. Had any of these antecedents been missing, the winning race-horse would not have won the race. Nor is this all. We have to include in our causes air, gravitation, and the fact that the earth is solid. No horse could win on turf which was based on vapour. But by all the thousands who witness a great race this whole mass of ulterior, though necessary, causes is ignored. The only causes which for them have any practical interest are those comprised in the organism of the winning horse itself. Who would contend that this horse had not won its own victory, on the ground that part of its own speed--a part which could not be calculated--was contributed by the crust of the earth, or the general constitution of the universe? Any one arguing thus would be howled down as a madman. Now, why is this? Why would the common-sense of mankind, in a practical matter like a race, instinctively exercise this kind of eclecticism, concentrating itself on certain causes and absolutely ignoring others? Such behaviour is not arbitrary. It depends on a principle inherent in all practical reasoning whatsoever. Let us see what this principle is.