icient presumption to warrant him
in examining whether there is any virulent ingredient in the milk.

Thus varying the circumstances so as to bring out a common antecedent,
though it does not end in exact proof, may indicate causal connexion
though it does not prove what the nature of the connexion is. Roger
Bacon's observations indicated that the production of rainbow colours
was connected with the passage of light through a transparent globe or
prism. It was reserved for Newton to prove by other methods that
white light was composed of rays, and that those rays were differently
refracted in passing through the transparent medium. We have
another example of how far mere agreement, revealed by varying the
circumstances, carries us towards discovery of the cause, in Wells's
investigation of the cause of dew. Comparing the numerous instances of
dew appearing without visible fall of moisture, Wells found that they
all agreed in the comparative coldness of the surface dewed. This was
all the agreement that he established by observation; he did not carry
observation to the point of determining that there was absolutely
no other common circumstance: when he had simply discovered dewed
surfaces, he tried next to show by reasoning from other knows facts
how the coldness of the surface affected the aqueous vapour of the
neighbouring air. He did not establish his Theory of Dew by the Method
of Agreement: but the observation of an agreement or common feature in
a number of instances was a stage in the process by which he reached
his theory.

III.--MILL'S "JOINT METHOD OF AGREEMENT AND DIFFERENCE".

After examining a variety of instances in which an effect appears,
and finding that they all agree in the antecedent presence of some one
circumstance, we may proceed to examine instances otherwise similar
(_in pari materia_, as Prof. Fowler puts it) where the effect does not
appear. If these all agree in the absence of the circumstance that is
uniformly present with the effect, we have corroborative evidence that
there is causal connexion between this circumstance and the effect.

The principle of this method seems to have been suggested to Mill by
Wells's investigations into Dew. Wells exposed a number of polished
surfaces of various substances, and compared those in which there was
a copious deposit of dew with those in which there was little or
none. If he could have got two surfaces, one dewed and the other
not, identical in