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.
When, with any practical purpose in view, we insist that anything is the cause of anything else, or produces anything else, we are always selecting, out of an incalculable number of causes, one cause or agency which, under the circumstances in view, may or may not be present; which a careless person may neglect to introduce; which an ignorant person may be persuaded to take away; or a recognition of which will influence human conduct somehow; while all other causes, which no one proposes to take away, or which no one is able to take away, are assumed by all parties, but they are not considered by anybody.