Similarly, earth moves towards the centre, fire turns away from the centre; fire causes heat, water causes cold, in accordance with a certain habit; but it is logically not impossible that a deviation from this habit should occur, namely, that fire should cause cold, move downward, and still be fire; that the water should cause heat, move upward, and still be water. On this foundation their whole fabric is constructed. They admit, however, the impossibility of two opposite properties coexisting at the same time in one substance. This is impossible; reason would not admit this possibility. Again, reason does not admit the possibility of a substance existing without an accident, or an accident existing without a substance, a possibility admitted by some of the Mutakallemim. It is also impossible that a substance should become an accident, that an accident should become a substance, or that one substance should penetrate another. They admit that reason rejects all these things as impossible. It is perfectly true that no notion whatever can be formed of those things which they describe as impossible; whilst a notion can be formed of those things which they consider as possible. The philosophers object to this method, and say, You call a thing impossible because it cannot be imagined, or possible because it can be imagined; and thus you consider as possible that which is found possible by imagination, not by the intellect, consequently you determine that a thing is necessary, possible, or impossible in some instances, by the aid of the imagination—not by the intellect—and in other instances by the ordinary common sense, as Abu Nasr says in speaking of that which the Mutakallemim call intellect. It is clear that they describe as possible that which can be imagined, whether the reality correspond to it or not, and as impossible that which cannot be imagined. This proposition can only be established by the nine aforementioned propositions, and no doubt these were exclusively required for the support of this proposition. This you will see clearly when I shall show and explain to you some important parts of this theory, which I shall now introduce in the form of a discussion supposed to have taken place between a Mutakallem and a philosopher.