The fundamental principle of the atheism of Spinoza is the doctrine of the simplicity of the universe, and the unity of that substance, in which he supposes both thought and matter to inhere.  There is only one substance, says he, in the world; and that substance is perfectly simple and indivisible, and exists every where, without any local presence.  Whatever we discover externally by sensation; whatever we feel internally by reflection; all these are nothing but modifications of that one, simple, and necessarily existent being, and are not possest of any separate or distinct existence.  Every passion of the soul; every configuration of matter, however different and various, inhere in the same substance, and preserve in themselves their characters of distinction, without communicating them to that subject, in which they inhere.  The same substratum, if I may so speak, supports the most different modifications, without any difference in itself; and varies them, without any variation.  Neither time, nor place, nor all the diversity of nature are able to produce any composition or change in its perfect simplicity and identity.

I believe this brief exposition of the principles of that famous atheist will be sufficient for the present purpose, and that without entering farther into these gloomy and obscure regions, I shall be able to shew, that this hideous hypothesis is almost the same with that of the immateriality of the soul, which has become so popular.  To make this evident, let us [Part II, Sect. 6.] remember, that as every idea is derived from a preceding perception, it is impossible our idea of a perception, and that of an object or external existence can ever represent what are specifically different from each other.  Whatever difference we may suppose betwixt them, it is still incomprehensible to us; and we are obliged either to conceive an external object merely as a relation without a relative, or to make it the very same with a perception or impression.

The consequence I shall draw from this may, at first sight, appear a mere sophism; but upon the least examination will be found solid and satisfactory.  I say then, that since we may suppose, but never can conceive a specific deference betwixt an object and impression; any conclusion we form concerning the connexion and repugnance of impressions, will not be known certainly to be applicable to objects; but that on the other hand, whatever conclusions of this kind we form concerning objects, will most certainly be applicable to impressions.  The reason is not difficult.  As an object is supposed to be different from an impression, we cannot be sure, that the circumstance, upon which we found our reasoning, is common to both, supposing we form the reasoning upon the impression.  It is still possible, that the object may differ from it in that particular.  But when we first form our reasoning concerning the object, it is beyond doubt, that the same reasoning must extend to the impression: