The tattvas are, in Indian philosophy, the fundamental principles or "thatnesses" from which the manifest world is composed; in the fivefold elemental scheme used in Tantra and Yoga they are the five gross elements, the pancha mahabhuta. Tejas or Agni is the fire tattva, the principle of heat, light, transformation and combustion, associated with the sense of sight and, in the subtle body, with the region of the solar plexus and digestion. In the classical Indian sources the five tattvas are a metaphysics of the elements, each with its subtle counterpart and its sense-faculty.
The element was later borrowed into Western esotericism: Rama Prasad's Nature's Finer Forces of 1890 introduced the tattvas to English readers, and the Golden Dawn adopted them as meditative symbols, giving fire the emblem of a red triangle. That Western use, from 1888 onward, is a modern overlay; the tattva doctrine itself is ancient and Indian, and fire's attributes of light and transformation belong to that older tradition.
Sources Classical Samkhya: An Interpretation of Its History and MeaningGerald James Larson with Nature's Finer Forces: The Science of Breath and the Philosophy of the TattvasRama Prasad