Shelley's "Defence of Poetry" is exactly that—a defense of the value of poetry to society. It was written in rebuttal to his friend Thomas Love Peacock's essay "The Four Ages of Poetry," a magazine piece which poked fun at poets for being useless in an age of scientific advancement (see the second reference link below). Shelley's way of arguing a case in favor of poetry is to endlessly expound upon the positive influences of poetry, rather than specifically undermining the value of scientific thought.
We do hear Shelley's thoughts on reason early in the essay, where he describes it as a rather benign counterpoint to artistry: "Reason is to imagination as . . . the shadow to the substance" (3). While this remark is as scathing as it gets, we can deduce Shelley's opinion on rational thought based on the fact that he apparently feels no need to speak further about it. He appears to be indicating, through his abundant examples of poetic virtue, that scientific, rational thought is essentially unimportant. By pointing out how fully poetry permeates the foundations of society, and by correlating poetry with societal success, Shelley implies that the lifeblood of humanity lies in artistic, passionate, pleasurable poetics, which fuel civilization perfectly well on their own.