If a loss would not have been suffered but for the commission of one or
more wrongs, each wrongdoer is jointly liable for the loss, even if in
respect of each wrong individually the loss would have been suffered
anyway because of the wrong of someone else. This is a rule of law. It is
nothing to do with what causation means.

So, if a builder contracts with A that he will supply him with bricks and
with B that he will supply him with mortar, if both in breach of contract
fail to supply the builder, he can recover against both for the loss
caused by not being able to build even though he would not have been able
to build without either the bricks or the mortar alone. That he cannot say
that either failure to supply individually caused him this loss, because
'but for' cannot be satisfied in relation to either A or B's individual
breach is true, but irrelevant.

So, in the two dog stabber case the result is clear, even if you think
(incorrectly in my view) that the only damages recoverable in the law of
torts are for consequential loss. No departure from 'but for' is necessary
to explain the result. (I confess that in my original example I assumed
that a dog which has been stabbed and doomed to die is now valueless. You
could have it stuffed I suppose.)

We are agreed, I think that both dog-stabbers have committed a tort. The
tort is fully constituted by stabbing the dog, just as the breach of
contract was fully constituted by not supplying the material. That we
cannot say as against any one dog-stabber "but for your stabbing, the dog
would not have died" is true, but completely irrelevant as a matter of
law. It just isn't the relevant legal question.

Further that we cannot say against either of them individually that their
stabbing resulted in the dog now being dead does not lead to the logical
absurdity of the dog's death being uncaused. The dog would not have died
but for both dog-stabbers, just as the builder's loss would not have been
suffered but for the failure to supply bricks and mortar. I don't reject
Mackie's aggregative move. I think he is plainly right. I just don't think
it is particularly helpful in understanding the law. I also don't think
lawyers have anything interesting to say to philosophers of causation.

Put another way, those who have focused solely on the defendant's
blameworthy conduct, and the current state of affairs which the plaintiff
now complains about, are ignoring the crucial legal question of whether
the defendant has wronged the plaintiff at some point in the past.

X caused Y entails "X was necessary in the circumstances for Y". My view
is that if you think your understanding of the law requires that that rule
of the universe has to be departed from, your understanding of the law is
wrong. Not the universe.

--
Robert Stevens
Professor of Commercial Law
University College London