David Hume believed that the ability to REASON is possessed by all humankind (assuming even rudimentary natural development… such as the mental powers of a two year old). Thus, the power of Reason, being shared by all, can UNITE all; it can provide common ground for interactions and, hopefully, agreements. If we did NOT share the same basic Reasoning faculty, then, writes Hume, “nothing could be more fruitless than to reason or dispute together.” (and, indeed, such activities seem fruitless enough already).
When we persist in disagreement it is due either to Reason-blinding prejudice or to a confusion concerning TERMS.
Nothing preserves a dispute so long (assuming open minds) says Hume, than “ambiguous expressions.” Mathematicians typically don’t dispute over equations because their Terms are well-defined. In math, “an oval is never mistaken for a circle.”
But of course, in the imprecise world of the everyday, there are no perfect circles. Unlike mathematics, the Terms of philosophical disputes are not always “clear and determinate” as they are in mathematics. In math, the smallest difference is immediately perceivable. Not so when it comes to the vague values assigned to Ideas. “The chief obstacle, therefore, to our improvement in the moral or metaphysical sciences,” writes Hume, “is the obscurity of the ideas, and the ambiguity of terms.”