If we are impressed with the idea that any one food does not agree with us, whenever we think of that food we contract, and especially our stomachs contract.

Now if our stomachs contract when a food that we believe to disagree with us is merely mentioned, of course they would contract all the more when we ate it.

Naturally our digestive organs would be handicapped by the contraction which came from our attitude of mind and, of course, the food would appear not to agree with us.
Take, for instance, people who are born with peculiar prenatal impressions about their food.

A woman whom I have in mind could not take milk nor cream nor butter nor anything with milk or cream or butter in it.

She seemed really proud of her milk-and-cream antipathy.
She would air it upon all occasions, when she could do so without being positively discourteous, and often she came very near the edge of discourtesy.