“We hold these truths to be self-evident…” But the truths in question are not self-evident, at least not to anyone prior to the Enlightenment, and not to many thinking men afterward.
Since the Founders considered their notions ‘self-evident’, they did not feel compelled to demonstrate them with any clarity. Hence, we cannot even affirm that they are ‘truths’ since we cannot be entirely sure what ideas are in question, other than a series of vague abstractions. We encounter terms like right, inalienable, and equality, whose meaning it is impossible to determine, and which are inevitably defined differently in the mind of every person who ponders them, and always to his own advantage.
One could argue, in Thomas Jefferson’s favor, that he had a sound defense of these principles in mind but did not have the room to write it in a document such as the Declaration, which was not a philosophical treatise. So be it. But where, then, in the writings of Jefferson or in any other founding father, can we find an adequate defense? We find that in virtually every case, these self-evident truths are not so much conclusions but are in fact the starting point.
Whether the founders arrived at them through carefully reasoned argument, or whether they were simply repeating John Locke, we cannot know, since they are not at pains to cite their sources.
The ideological founding of the American Republic was a grand instance of begging the question. A fallacy, in other words. One that has found almost global success.