Ask what yesterday is, and Emily Dickinson answers like a student reciting from a syllabus: it is History, it is Poetry, it is Philosophy. Then, in the second stanza, she pulls the desk out from under the lesson. Yesterday is also "mystery," and no one can say where it is "Today." The more cleverly we guess, the faster everything will "Flutter ... away." The central idea is that the past can be labelled in many ways but never actually located. Every attempt to pin it down only shows how little we hold.
The first stanza builds its confidence through repetition. "Yesterday is..." returns at the start of lines one and three like a formula, and each label changes how the past feels. As History, yesterday is "so far away," which turns time into geography, a country at the end of a long road. Poetry and Philosophy are different kinds of distance. Poetry reshapes what has gone into something felt, and philosophy asks what "gone" even means. The stanza closes on "'Tis Philosophy --" with a dash instead of a full stop, so the thought trails off into more thinking rather than arriving anywhere.
The sound of the poem makes this loss of footing easy to miss. The lines alternate between seven syllables and five, in a light, tripping beat that is mostly trochaic: YES-ter-day is HIS-to-ry. It almost sounds like a skipping rhyme. The rhyme is loose, too. History, Poetry, Philosophy and mystery all end on the same unstressed *-y*, a soft echo rather than a clean chime. The only true rhyme is "Today" with "away" in the second stanza, and "away" was already the last word of line two. The poem leaves and returns by the same word, but its meaning has changed. In the first stanza "far away" describes a fixed distance, and by the end "Flutter both away" describes movement, something escaping while we watch.
The word choices in the second stanza carry the poem's gentle irony. "Shrewdly" suggests sharp, calculating cleverness, a merchant's cunning. "Speculate" means to think, but judging by the sound of it beside "shrewdly," it may also hint at the gambler's or trader's sense of taking a risk for profit. Either way, the intellect is pictured as a keen dealer trying to strike a bargain with time, and it gets nothing for its effort. Against that heavy, abstract vocabulary, "Flutter" is a small, quick, living word. It brings to mind wings, something delicate and impossible to hold in a closed hand.
There is a useful ambiguity in "both." It could mean Yesterday and Today, in which case even the present, the only place we could stand to look back from, leaves with the past. It could also mean the mystery and our speculation about it. The poem does not decide. Whichever way we read it, we are left without solid ground.
A small detail in the version printed here points the same way. History, Poetry, Philosophy and Today are capitalised, but "mystery" is not. The first three are fields of study with names and shelves. A mystery is not a subject you can enrol in, and the lowercase letter makes it look smaller and quieter than the grand disciplines it defeats.
The poem matters because it fits a large question into eight short lines and then enacts its own answer. It is itself a piece of "Poetry," and it is also a piece of shrewd speculation about yesterday, so it cannot stand outside its own doubt. Dickinson, who lived in Amherst, Massachusetts, wrote in compressed forms like this one, with the dashes and capitals that mark her style. Here that style does real work. The dashes leave the thought open at the edges, and the capitals make the abstractions look solid just before they dissolve. Anyone who has tried to remember exactly how a day felt, and found only a few details left, will recognise what the poem describes.