e two essays, which are still in manuscript, one on substance and
accident, and the other called _Clavis Philosophica_. His chief work
appeared in 1713, under the title _Clavis Universalis_, or a _New
Inquiry after Truth_, being a _Demonstration of the Non-Existence or
Impossibility of an External World_ (printed privately, Edinburgh, 1836,
and reprinted in _Metaphysical Tracts_, 1837, edited by Sam. Parr). It
was favourably mentioned by Reid, Stewart and others, was frequently
referred to by the Leibnitzians, and was translated into German by von
Eschenbach in 1756. Berkeley's _Principles of Knowledge_ and _Theory of
Vision_ preceded it by three and four years respectively, but there is
no evidence that they were known to Collier before the publication of
his book.

His views are grounded on two presuppositions:--first, the utter
aversion of common sense to any theory of representative perception;
second, the opinion which Collier held in common with Berkeley, and
Hume afterwards, that the difference between imagination and sense
perception is only one of degree. The former is the basis of the
negative part of his argument; the latter supplies him with all the
positive account he has to give, and that is meagre enough. The
_Clavis_ consists of two parts. After explaining that he will use the
term "external world" in the sense of absolute, self-existent,
independent matter, he attempts in the first part to prove that the
visible world is not external, by showing--first, that the seeming
externality of a visible object is no proof of real externality, and
second, that a visible object, as such, is not external. The image of
a centaur seems as much external to the mind as any object of sense;
and since the difference between imagination and perception is only
one of degree, God could so act upon the mind of a person imagining a
centaur, that he would perceive it as vividly as any object can be
seen. Similar illustrations are used to prove the second proposition,
that a visible object, as such, is not external. The first part ends
with a reply to objections based on the universal consent of men, on
the assurance given by touch of the extra existence of the visible
world, and on the truth and goodness of God (Descartes), which would
be impugned if our senses deceived us. Collier argues naively that if
universal consent means the consent of those who have considered the
subje