Though we only hear directly about the speaker once in this poem, the poem's very first word is "I."
This means that, though we hear little about the personality of the speaker, the rest of the poem is in the light of his action (we're just assuming it's a he, since this is Wallace Stevens' poem). The jar, after all, didn't just magically appear on a hill in the middle of Tennessee. Our speaker put it there.
Even though the poem moves off to consider the jar, it's important not to forget the word "I." That's because it is from the point of view of this "I" that the wilderness seems to be controlled by the jar. Maybe, he has placed the jar in the wilderness as a sort of experiment. How, he may wonder, will this jar affect the wilderness around it? In the final stanza, we get our answer to that: though the wilderness comes under the reign of the jar, the jar will never actually be part of the wilderness.
So, this is essentially the result of the experiment that the speaker has conducted in this poem. By that token, we find it striking how Stevens himself, as the writer of the poem, is conducting a very similar experiment to the one undertaken by the speaker. The poem, in a way, represents a kind of investigation. What is the line between the natural and the man-made world? Is that line even an important one to draw? Stevens' writes his poem to think about those questions, in the same way that the speaker puts his jar on a hill in Tennessee.