There is a parallelism or identity of structure between the operations of the mind, as expressed in thought and language, and those of the reality which it grasps. in general the structure is that of 'unity-in-opposites.' this appears in many examples, static or dynamic, drawn from everyday life: 'people step into the same rivers, and different waters flow on to them; ' 'a road, uphill and downhill, one and the same; ' sea is water most pure and most polluted: for fish drinkable and life-giving, for human beings undrinkable and deadly.' these remarks and their generalizations are not meant to infringe the law of non-contradiction; rather they trade on it to point out a systematic ambivalence (between polar opposites) in the essential nature of things." -heraclitus (oxford companion to philosophy)
a. the structure of the world is that of unity in opposites.
b. the only constant in the world is change. so, the only constant in reality is change.
c. fire is the element that most accurately represents change.
d. therefore, the fundamental substance of which reality consists must be fire. this fire is spiritual in nature and is unlike ordinary fire but its nature is the same: constant change.