 Hume’s early studies of philosophical “systems” convinced him that philosophy was in a sorry state and in dire need of reform. When he was only 18 years old, he complained in a letter that anyone familiar with philosophy realizes that it is embroiled in “endless Disputes” (HL 3.2). The ancient philosophers, on whom he had been concentrating, replicated the errors their natural philosophers made. They advanced theories that were “entirely Hypothetical”, depending “more upon Invention than Experience”. He objects that they consulted their imagination in constructing their views about virtue and happiness, “without regarding human Nature, upon which every moral Conclusion must depend”. The youthful Hume resolved to avoid these mistakes in his own work, by making human nature his “principal Study, &amp; the Source from which I would derive every Truth” (HL 3.6).