Dickinson treats Memory here not as a river or a landscape but as a container, and the poem opens with the oldest problem of any container: it fills up. "When Memory is full / Put on the perfect Lid." The tone is a brisk household instruction, as if she were advising on jam jars. What she thinks deserves protecting is stranger. It is not a great event but "This Morning's finest syllable," a single unit of sound, the smallest piece of speech, spoken at the start of a day.
The central idea, as I read it, is that something excellent is easily spoiled by whatever comes after it. Once the mind is full, adding more does not enrich it. It crowds or cheapens what is already best. Evening is called "Presumptuous," a striking charge to bring against a time of day. It behaves like a latecomer with the nerve to speak after Morning has already said the best thing. The lid is the defence: seal the jar before anything else can be added.
The grammar leaves room for doubt. Lines three and four are inverted, with the object first ("This Morning's finest syllable") and the subject and verb after ("Presumptuous Evening said"). In ordinary order, Evening said the syllable. So the presumption may be repetition, Evening echoing and claiming what Morning originally gave until it wears thin. Or the syllable may simply be the best thing in memory, and Evening's talk the threat to it. The poem never chooses, and the reader has to hold both possibilities.
The construction is tight. The lines run roughly six syllables, six, eight, then six or seven, the compact, hymn-like shape Dickinson often used. The first two lines are short and snug, which suits an instruction about closing something. Line three then stretches to eight syllables, an irony in itself, since the longest line is spent on the tiniest thing. Line four snaps back. "Presumptuous" is the longest, most swollen word in the poem, and it sounds as pompous as the quality it names.
The rhyme is imperfect. "Lid" and "said" share their final consonant but not their vowel, so the chime is close without being exact. That seems fitting in a poem about a "perfect Lid," because the lid and its partner do not quite fit. The poem also ends on a dash rather than a full stop, and a dash on the second line as well. The advice is to seal the jar, but the poem leaves its own last line open, as though Evening's remark were still hanging in the air.
Dickinson's capitals do quiet work too. Memory, Lid, Morning and Evening all receive them, so a container and two times of day become characters in a miniature drama. Morning is the giver, Evening the interloper, Memory the vessel, and the Lid the one tool of control. It is a whole day compressed into four lines, from first word to last, with a small act of social rudeness at its end.
The "Lid" may also carry a darker shadow. Anything sealed away is protected, but it is also silenced. This is an inference from the imagery rather than anything the poem states, yet the word can bring a closed box or even a coffin to mind. The finest syllable is saved at the price of never being spoken again. The poem does not say whether that is a bargain worth making, and its brevity leaves the question unresolved.
Its significance lies in how fully it practises what it recommends. It values economy and restraint, the one perfect word over the crowd of ordinary ones, and it is itself scarcely more than two dozen syllables long. Instead of praising memory as a storehouse to be filled, it suggests that memory has limits and that the real skill is knowing when to stop. A reader can take it as advice about holding on to a good moment, or as a joke about talkative evenings, or as both. Few poems say so much about excess while using so little.