One interesting angle for an essay about this poem is the way that the inspiration and contributions of great thinkers are described. Spender uses natural (physical) as well as spiritual imagery to convey these ideas. This suggests that inspiration and the lasting impact of all thinkers is both material and abstract (metaphysical or even spiritual). The exact philosophy or spirituality of that abstract component is not described in particular or ascribed to a certain ideology or religion. This makes it general, universal, and accessible to many rather than a select group.
In the first stanza, the speaker describes the soul's history emerging from the light and song of the sun. Here, the sun is also described in terms of time (which could indicate the measurement of days by the sunrise). The sun as a source of life is also indicated as that which gives rise to the blossom of the branches and the blossom of desire. Here, a physical urge to grow (blossom of branches) is compared to a spiritual or humanistic urge (desire to tell the soul's history: something profound). And the sun is the light and song (source) of this desire/urge to blossom/tell:
Whose lovely ambition
Was that their lips, still touched with fire,
Should tell of the Spirit still clothed from head to foot in song.
In the second stanza, the speaker continues with natural imagery to describe the history of the soul, or the history of human culture. There is a history of great thinkers/thoughts from which inspiration and ideas can be drawn - as water is drawn from "ageless springs." The idea is that there will always be water coming up from the ground and, in human culture and history, there will always be a history of (sometimes embedded) great ideas waiting to be observed or dug out.
In the last stanza, the impact of the great thinkers is not described in terms of books, words, or even personal histories. It is described abstractly, with the imagery of nature and a hint of spirituality using the light and energy of the sun as a conduit. The "vivid air is signed with their honor." The great ideas of history are a part of the world in culture and books but also as part of a humanity's soul/history; so the speaker describes these ideas as imprints on the air, which implies a physical (talking about these ideas, passing them on) imprint and a vague spiritual one (ideas imprinting the air).