128 J.J. Haldane
as caused by that kind of agency; and second, those in which the argument
is from effects to a cause, itself then characterized simply as that which is
their source. Aquinas calls these demonstrations ‘propter quid’ (showing why)
and ‘quia’ (showing that) respectively; and he then goes on to write that ‘The
truths about God which St Paul says we can know by our natural powers of
reasoning – that God exists, for example – are not numbered among the
articles of faith, but are presupposed to them’ (Summa Theologiae, Ia, q. 2, a.
2 ad. 1).
Propter quid arguments are very familiar in the sciences and in everyday
causal reasoning. Suppose you notice a mark etched in the surface of a piece
of furniture and ask how it came about. Someone then points out that a glass
of whisky was previously lying there and explains that some must have spilt
and caused the stain due to the solvent power of its alcohol. The explanation
proceeds from a known cause to one of its effects showing why the effect
exists. By contrast consider the following case. Some while ago I noticed that
the electrical shower at home was running at a much higher temperature than
previously. This was a problem since it had become too hot to use, and so,
although I had no illusion that I could repair it, I did set to wondering why
the temperature had increased. The water was hotter and the flow was less;
and given the way in which electric showers operate – by running the incom-
ing cold water over an electrically heated element – these two factors seemed
likely to be connected. This phase of the reasoning was in part a case of
inferencepropter quid (inferring the increased heat from a knowledge of the
causal mechanism). What followed, however, was a demonstration quia; for
having reasoned that the temperature increase was due to reduced water flow
and having checked from other outlets that the water pressure elsewhere in
the house was normal, I inferred that there must be a partial blockage some-
where in the mechanism or in the pipe leading to it. I thus concluded ‘there
is an obstacle’.
Notice that this conclusion carries no more information than would have
been given by my saying ‘there is a something, I know not what, which is
such that it is reducing the water flow’ – to which, being Thomistically-
minded, I might well have added ‘et hoc dicimus impedimentum’, ‘and this we
call “a blockage” ’. Suppose, further, that this blockage is a small piece of
masonry wedged in the inflow pipe. My earlier reasoning demonstrated the
existence of this stone fragment not qua(as a) piece of masonry but simply as
an existing blockage. So we might say that I proved that there is a blockage
but did not show anything about its nature; after all being ‘a blockage’ is an
extrinsic characterization, in this case a description of the agent from its
effects (a blockage = that which blocks). In the terminology of the mediaevals,
which is once again current in philosophy, I have proved the existence of the
stonede re (the existence of the thing which is a stone) but not proved its