The Emmet's Inch & Eagle's Mile Make Lame Philosophy to smile.
This is one of the more difficult couplets. Like a lot of those surrounding it, it deals with knowledge and what we can and can't understand about reality.
An "emmet" is an ant, actually—so it only sees what's very small and close to the ground. An eagle, on the other hand, can see for miles. In comparison, Blake says that philosophy is "lame" (even though Blake is sort of a philosopher himself) because it's just a way of thinking about things, not a way of perceiving them.
The kind of philosophy you have is based on how much information you can take in. For instance, if humans didn't have a sense of smell, they wouldn't know that scents existed and wouldn't be able to reason about scents and compare them and so on.
So "lame philosophy" is smiling because it's realizing its own limitations—or, because it cockily can't see those limitations, or because it's realizing how everything depends on your perspective. The last bit, about everything depending on your own perspective, is similar to the ideas of the philosopher David Hume, who wrote about the limits of human knowledge (his life overlapped with Blake's, but Blake—being more mystical than the skeptical Hume—wasn't a fan; he definitely would've found Hume's philosophy "lame").
He who Doubts from what he sees Will ne'er believe, do what you Please.
This is another couplet that goes well with the one before it. The ant and the eagle both see reality—more or less of it, depending—and that's how they get information. They don't reason about it, they just see it directly.
They go from the gut.
But a doubter—unlike these animals—questions everything, and isn't left with anything to hang his or her hat on (except for doubt itself). Blake thought that everything we could see was, in some way, part of this greater spiritual reality—so, he takes it all fairly seriously and doesn't question its basic sense. Even someone's private fantasies and dreams are, for Blake, images of reality. He says, "Everything possible to be believed is an image of truth"—but this doesn't mean that some visions and ideas aren't more true than others.
So, Blake's saying that the doubter can't believe until he or she finally accepts that there's something beyond doubt, that there are images of truth in the human imagination and senses.