Recently, I've been testing a series of self-improvement CDs called Paraliminals, which claim to use state-of-the-art methods to give you, among other things, "instantaneous personal magnetism". The problem with evaluating them, though, is that you can't really go around asking friends and colleagues whether they think you've been demonstrating instantaneous personal magnetism over the past few weeks.

Actually, that's not true. You can. I did. Uniformly, they gave me a slightly scared look, which made it clear that they agreed I was indeed demonstrating a new personality trait, no doubt about it. Just maybe not the one I'd intended.

Paraliminals' selling point is that they're not meant to be hypnotic, but nor do you process them consciously. You can't: you're instructed to listen wearing headphones, and a syrupy-voiced American named Paul Scheele speaks two different scripts, one in each ear, at the same time. "Your conscious mind finds it difficult to process two voices simultaneously, so it shuts down," Scheele explains.

(Afterwards, I made the following transcript: "Your image of yourself and there was a special delight notice your potential has always been on occasion that image of you leaking springs and weedy patches...")

At first, it made me feel car-sick. But then further thought did become impossible, which is definitely relaxing, whether or not it instils the promised benefits (as well as Instantaneous Personal Magnetism, there are CDs called Ideal Weight, Anxiety-Free, Get Around To It and Positive Relationships, each for £15, from learningstrategies.com).

We're deep into the world of self-help "technology" here, so Scheele doesn't even try to claim support from peer-reviewed studies. This kind of thing bothers some people immensely, but as long as hideous amounts of money aren't involved, I'm disinclined to worry. If I spend £15 on a CD called Get Around To It, then have a productive few days, as I did, why should I mind if it was really the placebo effect, or if I was subconsciously trying to get my money's worth? Even if I'm still waiting for the personal magnetism to kick in...

Paraliminals makes much of being cutting-edge, but none of this would have surprised the French pharmacist Emile Coué, born 150 years ago. His 1922 book on "autosuggestion" is best remembered for advising people to stand at the mirror, repeating: "Every day, in every way, I'm getting better and better." Today, that's a pathetic cliché: if you actually said it, you'd feel an instant failure. But that hardly means that what we tell ourselves - or buy CDs to tell us - is irrelevant.

Coué used the example of a plank, 30ft long and a foot wide, placed on the ground: anyone could easily walk its length. "But now... imagine this plank placed at the height of the towers of a cathedral. Who then will be capable of advancing even a few feet?" The only difference is we imagine we can't. Coué concludes: "We who are so proud of our will, who believe that we are free to act as we like, are in reality nothing but wretched puppets, of which our imagination holds all the strings!"

oliver.burkeman@theguardian.com