Supervenience of a higher B on a lower A means that the same A entails the same B, or, alternatively, a difference in B for two things is due entirely to the difference in A in them. For example, some ethicists argue that moral propositions, such as “murder is wrong” supervene on the natural world. It’s a form of reductionism.
Let me define a relation called “subvenience” which is just like supervenience except that B is lower than A. I claim that the Aristotelian causes subvene on each other in a straightforward way.
The hierarchy is as follows: material, efficient, final, formal. Hence:
Probably the easiest subvenience to see that of the material cause on efficient. If X and Y have the same efficient cause, i.e., work or function in exactly the same way, then by that very fact they must have the same material cause, i.e., they are composed of the same stuff. Or: if any two objects are made of different matter (say, water and oil), then some of their behaviors will be different. (But not the reverse; e.g., the same metal can be used in both swords and plowshares.) How could we even say that X and Y are two different chemicals, say, if every test we ran on them produced the exact same results?
At the limit, if absolutely every behavior of X and Y were the same, then we’d have to conclude that X and Y are made of the same material.
Going a step further, if X and Y are, say, 2 identical wooden planks, then their having the same material and efficient causes does not guarantee that they will be used in the same way. For example, one plank can be made into a desk; the other, into a door. The reverse is more plausible. Or is it? Can’t a wooden desk and metal desk be equally serviceable to a person? Perhaps, and one can indeed be indifferent to what the desk he desires is made of. For example, a salesman in a furniture store can say, “We have two desks, but they are made of different stuff; one is woo…” The client interrupts: “Don’t even bother telling me; I don’t care.” By his own preference, the client demonstrates indifference.
Even if one disagrees, it seems possible for the customer to pick “at random” or based on an unrelated criterion such as which desk is shown to him first.
Again, one may buy a car which will serve him just as well whether the engine is made of steel or aluminum. For the practical purposes of an acting man, as long as, say, the gas mileage was the same, the two would engines would be completely interchangeable.
At the limit, however, if we (i.e., together as a race) could find absolutely no employment of X and Y at serving human ends wherein their efficiency or utility differed, then we wouldn’t be interested in differentiating their efficient causes.
Just as in first subvenience, we could not know if two identically working things are in fact composed of different substances, so in the second subvenience we would not care if the two identically used things worked differently. In both cases, the differences in the lower causes fade and become of no import upon the similarities of the higher causes.
Lastly, if any X and Y be designated formally the same (recall that the formal cause of X is the answer to the question “What is X?”), then their final causes are also identical, and through that, also efficient causes, and in their own turn, material causes. “What X is” includes into itself but is not limited to all the information provided by answering “What is X made of?,” “How does X work?,” and “What is X for?” Surely, we powerless to use in different ways any two things that are the same simply in every respect.
At the same time, all three answers may be the same for X and Y, yet their forms may still be different. One wooden door to the storage room could be 1 mm higher than the other yet secure the room and all that that implies equally well.