Newton's law of cooling makes the rate of heat loss proportional to the difference between:

1. A hot body cools faster when the surroundings are much colder. 2. The law makes that dependence exact. 3. The rate of loss of heat is proportional to the temperature difference of the body and the surroundings. 4. So the difference is between body and surroundings. _Source: syllabus Physics Part II, Ch 10 'Thermal Properties of Matter'_