THE HYPOTHESIS OF ETHER AND REID’S INTERPRETATION OF
NEWTON’S FIRST RULE OF PHILOSOPHIZING
ABSTRACT. My object is to question a recurrent claim made to the point that Thomas
Reid (1710–1796) was hostile to ether theories and that this hostility had its source in his
distinctive interpretation of the ﬁrst of Newton’s regulæ philosophandi. Against this view I
will argue that Reid did not have any quarrel at all with unobservable or theoretical entities
as such, and that his objections against actual theories concerning ether were scientiﬁc
rather than philosophical, even when based on Newton’s ﬁrst rule. I argue further that
Reid’s insistence on Newton’s rule concerns, not direct observation, but rather the notion
of explanation itself.
Thomas Reid called the ﬁrst of Newton’s four regulæ philosophandi a
“golden rule” and considered it to be “the true and proper test, by which
what is sound and solid in philosophy may be distinguished from what is
hollow and vain (Reid 1872, 236). By its means Reid made an effort to
clarify the difference between proper science and improper, between true
explanations and such that only seems to be so. Newton’s ﬁrst rule reads:
We ought to admit no more causes of natural phenomena than such as are both true and
sufﬁcient to explain their appearance.
This rule provides, according to Reid, two conditions which any account
or explanation of natural phenomena must fulﬁll. The ﬁrst condition, the
so called vera causa condition, states that the cause must be true, which
in Reid’s view means that the proposition(s) stating the law(s) of nature
must be true. The second condition, which may be called the sufﬁciency
condition, states that the particular phenomenon accounted for must be the
necessary consequence of the law(s) in question. To explain a phenomenon
of nature is to show it to be the demonstratively necessary consequence of
some known laws (Reid 1872, II chapter vi).
Reid’s insistance on the vera causa condition has generally been
taken to imply a limitation of the scientist’s choice of laws, theories and
hypotheses to such as involve no existential claims but what can be obser-
vationally veriﬁed. Such a view would accord with another salient feature
Synthese 120: 19–26, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.