As always, it is particularly important to look at quotations in context to help us be aware of what the author is trying to say by including them in their writing. In this case, if we have a look at the lines immediately after the quote you refer to, we can easily see the meaning that Emerson is trying to attain through talking about the "poetical sense" of looking at nature and the way that it contrasts with the normal human way of regarding the natural world:
It is this which distinguishes the stick of timber of the wood-cutter, from the tree of the poet. The charming landscape which I saw this morning, is indubitably made up of some twenty or thirty farms. Miller owns this field, Locke that, and Manning the woodland beyond. But none of them owns the landscape. There is a property in the horizon which no man has but he whose eye can integrate all the parts, that is, the poet.
To look at nature with a "poetical sense," therefore, is to see it in a way that "integrates" all of nature and does not regard nature in a way that is purely functional or seen in relation to man's posessions and interests. Emerson uses two examples where he talks about the wood-cutter who only sees a "stick of timber," whereas the poet is able to see the "tree." He then talks about farms that are measured and cut off from each other and owned by different families. The poet is the only person, Emerson argues, who is able to "integrate" all of these human divisions and see nature in its wonderful entirety.