How the thinking works
Reasoning is associative and heavily dependent on memory. This Mercury reaches conclusions by recognising a pattern it has lived through rather than by working forward from a premise, which makes it shrewd about people and less confident with anything abstract that has no precedent attached.
The recall is unusually complete and unusually selective at the same time. Emotionally weighted material is retained in detail; neutral material is dropped.
How it speaks and writes
Speech is indirect and careful, often approaching the point through a story. This Mercury reads the listener continuously and adjusts, which is why it is trusted with things people do not tell anyone else and why it can be hard to get a straight answer from.
Writing carries atmosphere well and argument poorly. Tone is the strength; the strictly logical paragraph is the part that has to be forced.
Where it goes wrong
Criticism is taken personally because it is stored personally. A technical objection is filed alongside the face of the person who made it, and both are still there years later.
The second problem is mood-dependence. This Mercury's assessment of a situation shifts with how it feels, and it will reason its way to whichever conclusion the current state supports without noticing it has done so.
Why the dignity is what it is
Mercury is peregrine in Cancer. The Moon rules the sign; Mercury holds no dignity and takes no debility here, so it works under lunar conditions rather than its own.
That is the whole of it. Peregrine means unhoused, not weakened, and the practical effect is that Mercury's information-handling gets routed through memory and feeling instead of through detached analysis. Dignity is a statement about fit between planet and sign, not a verdict on the person carrying it.
Against the opposite sign
Mercury in Cancer faces Mercury in Capricorn, which strips context out on purpose and reasons from structure. Cancer knows what a decision will do to the people in the room; Capricorn knows whether it is correct, and the two placements consistently underrate one another.
Taken as an axis they describe one problem. Cancer's development is usually the ability to hold a hard assessment without translating it into a personal one, and it tends to be learned rather than given.
Questions people ask about this
Is Mercury in Cancer a good memory placement?
For anything with feeling attached, yes, and in considerable detail. Neutral information is much less reliably kept.
Why does this placement take criticism badly?
Objections are stored with the person who raised them. The content is filed as an event rather than as data.
Why is Mercury peregrine in Cancer?
Dignity names fit, not quality. Mercury has no traditional standing in the Moon's sign, so it operates on lunar terms instead of its own.