The Romans examined chicken entrails. Private investors look at lists of directors' share dealings for portents - if only you read them right.

So what do we make of newly-knighted Sir Stuart Rose's £1 million splurge on Marks & Spencer shares yesterday, supported by three other M&S directors pitching in with the best part of another £300,000?

Either they borrowed the money, or this was personal, tax-paid cash, not an investment anybody makes to say "boo" to the stock market on a bad day.

It was ever going to shift the market. If you run a small AIM-listed company and the shares start sinking, you can probably stop the rot by buying up all the shares on offer for the price of a suburban house. David Montgomery, one-time editor of the Daily Mirror, did just that on Tuesday. He spent £400,000 buying shares in his media outfit Mecom after their price had almost halved on the day. They didn't bounce all the way back, but they did stop falling, then perked up a couple of pence yesterday.

A company the size of M&S is something else, though.

There are three explanations for Sir Stuart's £1 million plunge.

Possibly he had a rush of blood to the head - he very well might at the spectacle of M&S shares crashing by 19 per cent (£1.6 billion if everybody turned their paper loss into a cash loss).

After all, the Christmas sales he reported were up, not down if you count M&S's growing international operations.

Or else Sir Stuart may have felt he should be seen doing something as the shares sank, however briefly, below the 400p famously offered by Sir Philip Green and rejected by himself, until yesterday triumphantly.

That would be an admirable motive, but not in itself a reason to emulate him this morning.

The conclusion he would doubtless like us to draw, is that he saw the stock market's over-reaction as an irresistible investment opportunity, a chance to make a great deal of money, while proving himself right.

That may well be so. It would be in character. Unhappily it is not necessarily a cue to pile into the shares after him.

In particular, it would be wrong to imagine that Sir Stuart and his colleagues know something that nobody else knows. They had just put out a trading statement that was supposed to reveal everything relevant.

It would be absurd to suppose that they had kept some titbit of good news up their sleeve - not to say actionable to suggest it in print. It was a bleak document and you may be sure if there was anything cheery to say they would have said it.

None of that means that you should hold back if you are minded anyway to buy M&S for a recovery. This could be your chance. If there is no recovery, or it falters, Sir Philip might have another go.

That would be in character, too. Don't buy M&S, or any other share, just because you see the directors buying. Directors notoriously take a rosy view of the companies they run and cannot understand if the stock market fails to share it.

Just watch out when they start selling.