The sum of my answer and argument [i.e., to Hume] is, that although we know not the "secrets of nature," yet we know that nothing can "begin its own existence;" therefore there must truly be a "productive principle," a cause necessary for every new existence in nature;--that we gain the knowledge of a "necessary connexion between Cause and Effect," by an experimentum crucis, and therefore no greater number of invariable antecedents and consequents are wanted, than what is necessary, in order to observe what circumstances affect each other, or the contrary. That neither fancy nor custom creates the notion by an association of ideas; but the UNDERSTANDING gains it, by an observation of what is that circumstance, without which a new object does not exist. Things therefore could not change their places, nor nature alter her course, without a contradiction
Hence it is that a cause is wanted in the universe equivalent to the change from non-existence ot existence! And also that it is not more unreasonable to believe in miracles than in any other extraordinary phenomena of nature, when we may suppose, that efficient Causes have been in action, towards their production; and that final causes are of sufficient weight to justify the altered work of Providence!
An Essay upon the Relation of Cause and Effect, Chapter 2, Section 3, pp. 94-95.
Let it not be retorted, that it is easier to conceive of all the little changing beings we know of, as existing without a creator than of such a being [as God]; for I answer, it is not easier so to think; the one side of teh dilemma involves a contradiction, the other does not; the one is to image the existence of a series of dependant effects without a continuous being of which they are the qualities, and is equal to the supposition of the possibility of every thing springing up as we see it, from an absolute blank and nonentity of existence; the other is the result of referring like effects to like causes. The one is to regard each little being we know of, as the strange appearance of contrivance without design, and of being at once a series of changes in relation to no end, though apparently directed to it; the other is to believe in the infinite universe of mind, matter, space, and motion, eternally and necessarily existing: generating the creation of all minor existences in every form and kind that is possible, through the rounds of ceaseless time.
Essays on the Perception of the External Universe, Part II, Essay XI ("On the Immateriality of Mind"), pp. 391-392.
All changes are but the little beginnings of new forms of existence, derived from the Universal Essence which began not to be. All motions derived from previous motion form together but ONE ACTION put forth originally by the essential power to begin motion, itself no motion. To suppose otherwise, is to imagine it possible for all which we at present see, to be of itself capable of arising where there was nothing but a blank. The mind feels that such an hypothesis involves a contradiction; that the idea contains an impossibility.
EPEU, Part II, Essay XII ("On the Union of Mind with Organization"), p. 399.