Originalism's intellectual underpinnings are simple to state: the guiding principle is a quest for the original meaning of the document as applied to contemporary times. It's very different from textualism or literalism--under the former, Congress could regulate commerce between only two states (because the Commerce Clause provides Congress the authority to regulate commerce "among" the states) and under the latter, we'd have to have a constitutional amendment to have an air force (because the constitution doesn't contemplate the existence of aircraft). Whether it's easy to determine the original meaning of any particular constitutional provision is a different matter, but with Originalism, at least we know what we are looking for.
Not only are Originalism's intellectual underpinnings cogent, I would argue that Originalism is the only hermeneutic that is consistent with the point of having a written constitution in the first place. Any other interpretive method--pragmatism, libertarianism, liberalism, etc.--subverts constitutional principles for those called for by the method itself. In this way, all other interpretive methods are, at some level, themselves unconstitutional.
