Imagine you are walking down the street. It is a lovely day — clear blue sky, a light breeze, sunshine. You marvel at what people refer to as “the miracle of life”. The leaves are so green! The birds are chirping! You are blissfully caught up in a state of vitamin D-induced euphoria. You think of the person you love most in the world, someone to whom you are profoundly attached. Visualising their face, you have a sense of infinite safety; you are filled with joy.

Then, you picture them dead.

Suddenly you are filled with dread, consumed by all the what-ifs that are contained in this thought. Thousands of them, like a shoal of sardines, racingare contained in this thought. Thousands of them, like a shoal of sardines, racing frantically through your mind. The more you try to push this thought away, the more space it occupies, expanding uncontrollably until you are not just experiencing the loss but are fixating on the details. The funeral arrangements, the life-insurance papers, questions of their Pin and the password for their e-mail.

Forget the leaves on the trees — you have more pressing concerns. Where’s the joy in being alive now? The sunshine has become oppressive; it’s no longer energising; it causes melanoma. You cross over to the shady side of the street.

After a few deep breaths, you are able to control this thought, swatting it out of your head. There’s no point. You’ll deal with it when the time comes. You tell yourself: “Cross that bridge when you come to it.” Instead, you focus on how much you have to be grateful for. You remind yourself of the reasons you have to be happy. They outweigh the reasons to be unhappy, don’t they? They do. At least, that’s what you’re told. You are walking along again, feeling better. Borderline upbeat. You have your health, after all. If you have that, you have everything.

But what if you lose your health? Then what do you have? Not to mention the fact that money is tight. You can’t afford to have an illness. Stop. Thinking. This.

You tell yourself you don’t have cancer, multiple sclerosis, glaucoma… You don’t have lupus. You remember that the other day you read an article about a cancer drug that shows promise against lupus. You felt at ease after you read this, knowing that it’s out there — just in case. You are feeling euphoric again, believing it will be okay. You look at your watch. You’ve been walking for 15 minutes. That’s enough sunshine.

Now, imagine this. Maybe this walk has been productive. Imagine that, after indulging in all the what-ifs and worst-case scenarios, rather than returning home in a state of heightened anxiety, you are actually refreshed and calm. You have sweated out all negativity and prepared for any eventuality. It would be easier not to have to do all these mental gymnastics, but if you are limber enough to handle it, what’s the harm?

There are benefits to being a pessimist, by which I mean someone who expects the worst possible outcome — not someone who promotes failure. Pessimists tend to be worriers, and worrying is not necessarily a bad thing. There is a significant difference between the pessimistic person who worries about the worst-case scenario but is willing to give it a shot, and the pessimist who assumes that they will fail, that it won’t work out, so they won’t bother.

The latter is associated with “hopeless pessimism”, and with depression. In contrast, there is “defensive pessimism”, which is not related to depression and may even be a protective factor. Dr Julie Norem, professor of psychology at Wellesley College, Massachusetts, addresses the phenomenon in her book The Positive Power of Negative Thinking.

“If you are hopeless because you don’t see any way that you can do what you need or want to do, there’s no motivation to try. Defensive pessimists, on the other hand, can envision plenty of ways that things can go wrong, but they also have some confidence in the possibility that they can do what needs to be done as long as they’ve prepared for all the bad things that are likely. The most important thing about their strategy is that they keep trying, working, moving forward, despite their anxieties and worries.”

I consider myself to be in this category. I am a functional pessimist, examining all the what-ifs in any situation and fretting about every possible outcome. I assume I won’t succeed, but am not so convinced of it that I’m unwilling to try new things, albeit with a negative attitude, hesitant to believe I will enjoy them — as, usually, I don’t.