See Leviathan, Chapter V : Of Reason and Science :
when we reason in words of general signification, and fall upon a general inference which is false; though it be commonly called error, it is indeed an absurdity, or senseless speech. For error is but a deception, in presuming that somewhat is past, or to come; of which, though it were not past, or not to come, yet there was no impossibility discoverable. But when we make a general assertion, unless it be a true one, the possibility of it is inconceivable. And words whereby we conceive nothing but the sound are those we call absurd, insignificant, and nonsense. And therefore if a man should talk to me of a round quadrangle; or accidents of bread in cheese; or immaterial substances; or of a free subject; a free will; or any free but free from being hindered by opposition; I should not say he were in an error, but that his words were without meaning; that is to say, absurd.
We may say that an error comes from asserting something like: "Today is Monday" or "My name is Paul" that is "factually" wrong but not impossible, while an absurdity is a contradiction, like: "A round square".
An absurdity is a sort of "error of computation", due to a mistake ina pplying the logical rules. The result is not a false assertion, but something meaningless (compare with Vienna Circle's criterion of cognitive significance and the related rejection of metaphysics).
We can see also Hobbes: Reasoning as Computation :
For Hobbes, the mind contains sense, imagination, and the workings of language, and no further rational faculty, such as the Cartesian immaterial mind that can grasp natures by clear and distinct perception. [...] For Descartes, sense and imagination are, as in Hobbes's story, closely connected to the workings of the brain, but higher cognitive functions are performed by the immaterial mind. Hobbes denies the existence of that immaterial mind, and needs other accounts of those functions.
Hobbes describes reasoning as computation, and offers sketches of the computation that he thinks is going on when we reason.