"Three years she grew in sun and shower," "Strange fits of passion have I known," "She dwelt among the untrodden ways," "A slumber did my spirit seal," and "I travelled among unknown men" are often grouped together as the "Lucy poems." The identify of Lucy is uncertain.
These poems praise Lucy and mourn her death. They are love poems and eulogies. In the first stanza of "Three years she grew," the speaker has Nature personified and speaking:
Then Nature said, "A lovelier flower
On earth was never sown;
This Child I to myself will take;
She shall be mine, and I will make
A Lady of my own.
Nature has claimed Lucy, compared to a flower, at the age of three, and will make her into a lady. Nature continues to say how Lucy will develop into maturity. Nature will be both "law and impulse"; that is, Nature will watch over Lucy and guide her with law (reason and/or restraint) as well as with impulse (emotion and/or desire). Thus, Nature will be like Lucy's guardian angel ("an overseeing power / To kindle or restrain) who will oversee and guide Lucy in reason and emotion. In the subsequent stanzas, Nature explains how she (Nature) will guide and protect Lucy as she grows up:
Nor shall she fail to see
Even in the motions of the Storm
Grace that shall mould the Maiden's form
By silent sympathy.
So, the laws of nature and the beauty (impulse) of nature will give Lucy a sense of belonging, as if Nature itself is her guardian/parent.