Mercury is how you think and how you speak. Not what you think, that is the rest of the chart, your history and your choices. Mercury is the format: the speed, the way of organising, the manner of saying, and also of understanding what is said to you.

And here is what almost no text explains, though it is the most useful fact about this piece: your Mercury can only be in three signs, your Sun's, the one before or the one after. Mercury never moves more than 28 degrees away from the Sun, and a sign has 30. There is no Sun in Aries with Mercury in Leo, and no calculator will say otherwise.

That is what answers the question that comes up all the time: why two people of the same Sun sign think in such different ways. Sun in Aries with Mercury in Aries decides on the spot and says what it decided; Sun in Aries with Mercury in Pisces decides on the spot and cannot explain why; Sun in Aries with Mercury in Taurus looks impatient and takes three days to answer an email.

Having Mercury in Aquarius means you think for yourself and arrive at the answer from outside. The head in Aquarius rises a few metres and looks at the whole system: not the part, but how the parts fit together. And from there it sees the solution nobody saw, because nobody was looking from that angle.

Mercury is the planet of thought and speech; in Aquarius, it gains distance and independence. This person mistrusts consensus by reflex, does not accept an argument from authority and prefers understanding the principle to memorising the procedure.

And here is what hardly any text writes down: they do not disagree in order to be contrary: they genuinely did not get there. Their reasoning started from another point and followed another path, and that is why the conclusion comes out different. When somebody accuses them of being contrary, it is usually because they did not follow the route, and they hardly ever have the patience to retrace it out loud.

There is a second piece that changes everything: which house this Mercury fell in, the area of life in which this head works. Mercury in Aquarius in the 11th house doubles the subject: somebody who thinks in collectives and in futures. In the 4th, somebody whose family never quite understood how this head works. The sign says HOW; the house says WHERE.

It shows in how often this person reaches the same place by another route. These are concrete signs, the kind you recognise in your own week:

• solves the problem from an angle nobody had considered
• mistrusts what everybody accepts, by reflex and not as a pose
• understands systems faster than they understand people
• explains clearly and with a coolness that sometimes sounds like disinterest
• loses interest when the subject becomes routine
• has friends from completely different worlds, and all of them find them odd

Notice what this list does not say. It does not say the person is cold nor contrary, two labels this position carries and which describe the excess, not the position. It says their thinking is independent and systemic.

The strength is originality: this person finds the way out that was not on the map. They think outside consensus effortlessly, and that is what makes them useful exactly when the known method has already failed and everybody is repeating the same path harder.

In practice that turns into three useful things: a capacity to abstract that serves in technology, science and any system; an intellectual independence that stops them boarding a bad idea under group pressure; and a view of the future that has them seeing the trend before it becomes consensus.

That strength does best in technology, science, research, a collective cause, innovation, engineering and any work where thinking differently is the job. And it does badly in a very hierarchical environment, in an arrangement demanding that a procedure be repeated without understanding it and in any place where disagreeing is treated as insubordination.

There are two, and they are opposites: the excess is independence turning into intellectual stubbornness, and the deficiency is whoever was convinced they think wrongly and started imitating another Mercury. The Shelly Method reads every archetype through four faces, the light, the shadow, the excess and the deficiency, and it is the last of them that hardly any text mentions.

The excess is disagreeing on principle: when the consensus is right, the person looks for the hole until they find one, and defends the exception as if it were the rule. It also shows up as emotional distance in conversation, they analyse the problem of somebody who is crying, and the analysis is correct and was not what the person needed.

The deficiency is the opposite and hurts more, and here it has a recognisable origin: school rewards two Mercuries and punishes the other ten. It rewards Taurus and Capricorn: obedience to method and to order. A Mercury in Aquarius that took a beating for being odd learns to repeat what is expected, to hide the different idea and to call its own clarity arrogance, and becomes somebody bitter, who still sees the way out and no longer says it.

Both faces have the same raw material, and that is why the repair is the same: an independence that needs to be translated, and not hidden. It is not solved by asking the person to adapt more, they dim themselves, and the attempt produces the deficiency described above. It is solved by their showing the route, and not only the conclusion, and that is what Mercury's house and the rest of the chart show.

It matures when the person discovers that a good idea has to be explained in order to exist, and this discovery is worth more than any attempt to learn to fit in. Fitting in by effort almost always fails, and produces the deficiency described above.

There is a rhythm that helps understand the arc, and it belongs to Mercury itself: three or four times a year it goes retrograde, for about three weeks. It is not a punishment: it is the period in which thought naturally goes back, revises, rereads, finds again. For whoever has Mercury in Aquarius, those weeks stir technology and groups: the system that goes down, the misunderstanding in the group chat, the collective conversation that has to be had again. And if you were born with Mercury retrograde, and about eighteen per cent of people were, that is your way all year round: saying, hearing it back and revising until you get where you were going.

Maturity here is not learning to agree. It is the person coming to show the path to the disagreement. A mature Mercury in Aquarius goes on arriving from outside, and starts bringing the others along, and it is that translation that turns oddness into contribution.

No honest text gives an age for this. The arc depends on the whole chart and on the life the person had.

It says how you think, and not what you think nor how much you know, and that distinction is the one popular astrology makes worst. Mercury is the format; the content is your life.

The house this Mercury fell in changes the area in which this head works: Mercury in Aquarius in the 6th house is somebody who reinvents the whole work process; in the 7th, somebody who needs a partner to translate what they meant. See the series The Sun in the 12 houses to understand what each house means.

Your Sun's sign is next to this one or the same, and the combination of the two is what produces the difference you feel in relation to other people of your sign. See the series The Sun in the 12 signs.

And there is the Moon, which says what you need inside. Mercury in Aquarius with a Moon asking for the opposite produces somebody who thinks one way and feels another, and it is that mismatch, and not the sign on its own, that usually explains the complaint. See the series The Moon in the 12 signs.

A limit said out loud, because it is also part of reading seriously: none of this is a diagnosis. Astrology describes tendency and gives vocabulary for what the person already lives; it does not measure intelligence, does not identify a learning difficulty and does not replace a professional assessment.

And the questions that come up most on this subject, with the short answer: