• identified with certainty. There aren't any general rules. There aren't any shortcuts. The name of the mushroom must be known.
• tolerated by most people. A good field guide passes on the experience of people who have eaten particular species.
• found in a wholesome environment. Mushrooms can absorb herbicides and heavy metals.
• fresh. Rotten food is never edible.
• cooked. Heat softens indigestible mushrooms. It may vaporize some poisons and reduce the potency of others.
• eaten in reasonable quantities. Some mushrooms are OK in small portions but troublesome when overeaten. And there's always the risk of a good old- fashioned bellyache.
• eaten by healthy adults. Children, old people, and ill people may be sickened by mushrooms that are enjoyed by others.

Some people get sick anyway. Alcohol combined with certain species causes illness. A few people are sickened by allergies or unusual sensitivity. Be kind to your doctor–don't confuse him by eating several species at one sitting. Experts can help, but eating mushrooms (or any food) can never be entirely safe.