In "Preface to Lyrical Ballads," Wordsworth argues that poetry is more closely united with the universal truth of human experience than with any other discipline (subject). He contrasts this truth of human existence (via poetry) with the truth/knowledge gained by scientists, botanists, and historians. Wordsworth notes that Aristotle describe poetry as the most philosophical of all writing because poetry's object is general truth and experience, whereas the scientist's object of truth is a particular thing such as the function of an electron.
In this preface, Wordsworth notes that poetry is, itself, timeless because its object can (or should be) the human experience.
Emphatically may it be said of the Poet, as Shakespeare hath said of man, ‘that he looks before and after.’ He is the rock of defence for human nature; an upholder and preserver, carrying everywhere with him relationship and love. In spite of difference of soil and climate, of language and manners, of laws and customs: in spite of things silently gone out of mind, and things violently destroyed; the Poet binds together by passion and knowledge the vast empire of human society, as it is spread over the whole earth, and over all time.
Wordsworth goes a step further and attempts to write in a more common language, even using prose rather than traditionally metered poetry, in order to appeal to all readers (not just poets). Wordsworth believes that inasmuch as poetry is the timeless practice of tapping into general truth, it must be a practice applicable to all people (not just poets).
It has often been said that Wordsworth wanted to illuminate the extra-ordinariness of ordinary experience. Ordinary experience is the first and last of all knowledge. (Biology, for instance, is a particular knowledge.) Poetry is as immortal as the heart of man because man (humanity) will always have a general curiosity and intuition about existence, feeling, suffering, and pleasure. And poetry, according to Wordsworth, will always be (or should be) the most direct and general expression of these fundamental conditions of human experience.