In the "Preface to Lyrical Ballads" Wordsworth discusses the pleasure we receive from the deployment of regular metrical language in a poem. This is Wordsworth's way of reassuring his readers that although his conception of poetry is indeed revolutionary in terms of its language and subject matter, it will not depart from the tried and trusted principles of regular poetic meter.

According to Wordsworth, metrical regularity isn't just something that gives order and stability to a poem. It also gives its readers pleasure. The mind is able to derive from regular meter what Wordsworth calls "the perception of similitude in dissimilitude". Broadly speaking, Wordsworth means by this that all pleasure—whether it's the pleasure one gets from poetry, or from sex—is generated by a combination of contrasts.

For instance, in Wordsworth's day the only societally sanctioned sexual relations were heterosexual, that is, those based on the contrast between a man and a woman. And in terms of Wordsworth's poetry there is also a contrast between the radical subject matter and language he uses—which pertains to ordinary folk—and the regular meter in which he writes such poetry. In either case, the perception of similarity in difference, and vice versa, gives us pleasure.

It is this principle, the pleasure that the mind derives from the perception of similitude in dissimilitude, that is the "great spring" of our mind's activity. It is in this great spring that the sexual appetite and all its associated passions, not to mention the passions of the mind, find their ultimate source.