Wordsworth sees Nature as, in some sense, a projection of the mind of man. This is typical of Romanticism, with its focus on the inner self, its perception of man as a kind of godlike being, and its concept of the literal outer world as in some way an illusion, a cover of the ultimate reality that lies beneath it. Wordsworth doesn't explicitly or directly express this Kantian philosophical idea that was "in the air" at the time, but it is implied by much of his poetry.
In what might be his most famous work, the "Intimations of Immortality" Ode, Wordsworth links his sense of self (and the immortality of his psyche) to the outside world. As a child, he communed directly with nature and felt something magical and eternal in it; as an adult, he has lost the immediacy of this feeling, but through the remembrance of childhood, he is able to console himself that these "intimations" were real and valid:
Though nothing can bring back the hour
Of splendour in the grass, of glory in the flower,
We will grieve not, rather find strength in what remains behind.
Nature represents a primal innocence that is lost when "the world"—that is, the world of mundane human activity—becomes "too much with us." The poet laments that
The sea that bares her bosom to the moon,
The winds that will be howling at all hours,
And are up-gathered now like sleeping flowers;
For this, for everything, we are out of tune.
These things from which modern man in his maturity (both the maturity of adulthood as well as that of the modern, mechanized age) has become disconnected are not only the things of the natural world, but the older, mythic concepts in which man no longer believes. Wordsworth's wish is that he might
Have sight of Proteus rising from the sea,
Or hear old Triton blow his wreathed horn.
In a former time, man's concepts, just like those of a child in the modern age, were linked to the outer, natural world. Mythology—as in the examples Wordsworth gives at the end of his sonnet—was an outgrowth of Nature, which is itself a part of man's mental cosmos. Much of Wordsworth's poetry is filled with regret, expressing the loss of this innocent union between mind and matter.