The story goes that Mrs Descartes was throwing a New Year's party to celebrate the arrival of 1630 and had spent weeks preparing. She had invited all the local jet (equestrian?) setters.
Moments before the guests started to arrive she instructed Descartes that the pastries on the table to the side were not to be eaten until after midnight to make sure there was enough food to keep the guests from leaving too soon. To make sure, she tasked Rene with the job of guarding them until an hour or so after midnight at which time she would invite the guests to help themselves. Though deep in thought, he agreed to mind the table.
As the party got into full swing, Descartes found himself in an absorbing philosophical discussion with Vandyke over why Titian removed a church from the Venetian background in one of his paintings. To hear each other better, the two wandered away from the crowd, in the direction of the forbidden baked goods.
Without Descartes noticing, Vandyke starting munching thoughfully on a pastry. Suddenly Descartes snapped out of his thoughts and realised what Vandyke was doing. His reaction surprised Vandyke who figured that Descartes surely must have just thought of something of great significance. Discretely, Descartes wrote a message on a napkin and handed it to Vandyke so as not to attract his wife's attention. However, just at that moment they were interrupted, which meant Vandyke could only stuff the napkin into his pocket for later.
The next morning he removed the napkin to see what profundity his friend had bequeathed him and, sure enough, there scrawled in Descartes hand was an expression of timeless insight, "I think they're for 1 am."