 A second unified view of causation in the law is the oldest of these kinds of proposals. It conceives of causation as a metaphysical primitive. Causation is not reducible to any other sort of thing or things, so there is little by way of an analysis that one can say about it and so very little that juries should be told about it (Smith 1911). The one thing we can say is that the causal relation is a scalar relation, which is to say, a matter of degree. One thing can be more of a cause of a certain event than another thing. Given the scalarity of causation, all the law need do is draw the line for liability somewhere on the scale marking degrees of causal contribution. On matters that vary on a smooth continuum, it is notoriously arbitrary to pick a precise break-point; where is the line between middle age and old age, red and pink, bald and not-bald, or caused and not-caused? This approach thus picks an appropriately vague line below which one’s causal contribution to a given harm will be ignored for purposes of assessing responsibility. Let the defendant be responsible and liable for some harm only when the degree of his causal contribution to that harm has reached some non de minimus, or “substantial”, magnitude. This is the original “substantial factor” test, as articulated by Jeremiah Smith in 1911. To the common objection that the test tells us little, its defenders reply that that is a virtue, not a vice, for there is little to be said about causation. Like hard-core pornography, causation is something we can “know when we see it” (Potter Stewart’s language about pornography in Jacobellis v. Ohio), without need of general definitions and tests (Borgo 1979).