Next morning, the quarrel is made up. Johnson owns that he spoke in passion, Boswell that he took it too hard, and they set out in a boat for Skye.

In most of The Journal of a Tour to the Hebrides with Samuel Johnson it is Johnson who is the star, Boswell the roadie, Johnson the sage, Boswell merely his amanuensis. It is clear who is dependent on whom. Yet in this incident, the roles are suddenly reversed.

This is depression at work, a condition almost always marked by the fear of abandonment. (Whether early experiences of abandonment ’cause’ depression can be left moot.) It is dusk, a bad time (Lowell’s ‘skunk hour’) in the daily mood pattern. Johnson maintains a ‘dreary silence’ and ‘seems to be meditating’. All in all, if I am right, he is in an extremely vulnerable state. Boswell’s sudden, unexplained departure fills him with fear and enrages him. Their friendship has given him not the least ground to suppose that Boswell will behave unpredictably (‘one cannot tell what he may do next’). In that phrase we hear the formless panic of depression.

Although the rage subsides, he is still angry later that night.

Sir, had you gone on, I was thinking that I should have returned with you to Edinburgh, and then have parted from you, and never spoken to you more.

Neither man invokes depression under its 18th century names (‘spleen’, ‘hyponchondria’) to account for this incident. And obviously there are other possible readings of Boswell’s account. What makes me more confident of mine, however, are two Latin poems Johnson composed later that week.

The first is an Ode to the Isle of Sky. (I quote the English versions made later.)

And yet to climb the hilly heath,
Or search the hollowed cave beneath,
Or count the white waves as they flow,
Affords no cure for mental woe.

The storms that shake the troubled soul,
‘This thine, Almighty, to control;
And, as thy wise decrees dispose,
The tide of passion ebbs and flows.

A second Ode, a few days later, is addressed to Hester Thrale.

Whether she sooths her husband’s toils,
Or spreads her fond maternal smiles,
Or with a book the hours beguiles
Her fancy to regale;

May she of me be mindful found!
May faith with mutual faith be crowned!
So shall the shores of Skie resound
The gentle name of Thrale.

He wishes to know that when they are separated, he and this woman on whom he has come to depend so much, that he is not gone from her mind, that their ‘mutual faith’ is sustained.

This sequence of moods, feelings and actions on the Tour shows one way in which Johnson’s depression is woven into the texture of his everyday experience. We are so accustomed to thinking of him as a granitic block that to imagine him as needy and dependent requires some adjustment. Needy and dependent, however, he certainly was, a fact about him recognised more clearly by women – Fanny Burney, Hester Thrale – than by men. Johnson names and knows what ails him, and is familiar with many of its effects. Perhaps by ‘passion’ he meant, not just strong emotion, but also the effects of illness, and means Boswell to pick this up. If so, Boswell didn’t.

The serotonin transporter gene, 5HTT, also has two alleles, and is known to be involved in mood. Moffitt and Caspi found that people with one allele were 2.5 times more likely to develop clinical depression than those with the other – but, again, only under particular circumstances. The risk applies only to people who also experience stressful life events such as unemployment, divorce or bereavement. When their environments are happy, their genotypes made no difference.

The ghost of Samuel Johnson whispers:

Yet hope not life from pain or danger free,
Or think the doom of man reversed for thee

The reality conveyed by the letters is completely different. Seems I did write, and here’s part of the response:

Believe me, I’ve no regrets, only a great deal of happiness to look back on.

There are more letters. Seems also that I was exploring the possibility of leaving for her city – permanently? – and although she was now involved with someone else, she was busy organising a job and somewhere to stay. Kindness and care, when all I had remembered was rage and betrayal.

No need to go into the psychodynamics of that, which are familiar enough. It’s typical of depressive thinking. In deep depression, as many of us know, we are the worst villains that ever crawled between earth and heaven, everything in our past is a crock of shit. When depression is under control, which mine mostly is, that’s easy enough to understand and to watch out for. Even when you’re some way down, you can say to yourself, here come the demons again, they’ll pass, the lying twisters. A measure of deep depression is the complete loss of that perspective.

That’s the general picture. But there’s a somewhat more subtle difficulty. In health you know damn well that you’re just another everyday sinner, and you can look at the past without falling over backwards. Just under the surface of your memories, though, there are these engraved patterns, and unless something jogs you to revisit them, there they stay. This is in all the textbooks – that you’re supposed to ‘work through the implications’ of your condition. I guess that’s what people in interminable therapy are trying to do.

For many years, I’ve carried around this little chunk of guilt quite needlessly. How many more are there? Instead of a suitcase, how I wish now I had a roomful of stuff.