It is important to Hume's program in the remainder of the Enquiry that he establish this thesis. Indeed this is the cornerstone of Hume's method of analysis of metaphysical concepts. He thinks this method can be used to settle important disputes. Since impressions are (Hume claims [text]) more lively and determinate than ideas, he thinks that reducing a problem from one about ideas to impressions (as his thesis would encourage), makes it easier to solve. This optimism can be seen in the final paragraph from the section "Of the Origin of Ideas." [text]
Later, in section VII, Hume returns to this optimistic description of the import of this thesis:
|It seems a proposition, which will not admit of much dispute, that all our ideas are nothing but copies of our impressions, or, in other words, that it is impossible for us to think of any thing, which we have not antecedently felt, either by our external or internal sense. I have endeavoured to explain and prove this proposition, and have expressed my hopes, that, by a proper application of it, men may reach a greater clearness and precision in philosophical reasonings, than what they have hitherto been able to attain. Complex ideas may, perhaps, be well known by definition, which is nothing but an enumeration of those parts or simple ideas, that compose them. But when we have pushed up definitions to the most simple ideas, and find still some ambiguity and obscurity; what resource are we then possessed of? By what invention can we throw light upon these ideas, and render them altogether precise and determinate to our intellectual view? Produce the impressions or original sentiments, from which the ideas are copies. These impressions are all strong and sensible. They admit not of ambiguity. They are not only placed in a full light themselves, but may throw light on their correspondent ideas, which lie in obscurity. And by this means, we may, perhaps, attain a new microscope or species of optics, by which in the moral sciences, the most minute, and most simple ideas may be so enlarged as to fall readily under our apprehension, and be equally known with the grossest and most sensible ideas, that can be the object of our enquiry. (Section VII: "Of the Idea of Necessary Connexion" paragraph 4)|
Hume then later in the Enquiry uses this method to explore the idea of necessary connection and one can see here his reliance on his thesis: "To be fully acquainted, therefore, with the idea of power or necessary connexion, let us examine its impression . . ." (Section vii, paragraph 5) If the idea need not be derived from impressions, Hume may be looking in the wrong place for an understanding of it.