How the thinking works
Thinking is confident and organised around a position. This Mercury forms a view and then reasons from it, which is not the textbook order but does produce clear, memorable arguments. Complexity is handled by simplifying it into something that can be stated.
Learning works best when there is an audience, real or imagined. Explaining a subject to someone else is the mechanism by which this mind actually understands it.
How it speaks and writes
The voice carries. Timing is instinctive, the vocabulary is generous, and the effect on a room is real even when the content is ordinary. This is a natural teaching placement and an even more natural persuading one.
Writing has personality and pace, with a weakness for the phrase that sounds better than it is accurate. The best editing this Mercury receives is from someone unimpressed by it.
Where it goes wrong
Because ideas are held as personal property, being contradicted registers as being diminished. This Mercury will defend a weak position longer than the evidence justifies, and the retraction, when it comes, tends to be quiet and unannounced.
The other cost is oversimplification. A thing that cannot be said impressively risks being restated until it can be, and some accuracy is lost in the process.
Why the dignity is what it is
Mercury is peregrine in Leo. The sign belongs to the Sun, and Mercury has no rulership, exaltation, detriment or fall here.
Without its own footing, Mercury takes on solar habits — communication becomes an act of identity rather than an exchange of information. That is a real difference in behaviour and not a defect. Dignity describes the fit between what a planet does and what a sign asks of it, which is a separate matter from how well anyone thinks.
Against the opposite sign
Mercury in Leo faces Mercury in Aquarius, which detaches an argument from the person making it as a matter of principle. Leo cannot fully separate a view from the self holding it; Aquarius cannot see why anyone would fail to.
The pair is one axis. Leo's useful correction is the Aquarian habit of testing a position it would prefer to be true, and it is generally acquired the hard way, in public.
Questions people ask about this
Is Mercury in Leo arrogant?
It is invested. A view is held as personal property, so disagreement is felt more sharply than the content of the disagreement warrants.
Is this a good placement for teaching?
Unusually so. Explaining something aloud is how this Mercury understands it, and the delivery holds an audience.
Why is Mercury peregrine in Leo?
Dignity names fit, not merit. Mercury has no traditional dignity in the Sun's sign, so it borrows solar habits of expression.