Buddha denied permanent reality to anything that is perceived in this world. Later Buddhist philosophers developed this into a regular school called ‘kṣaṇika-vāda’ or theory of momentariness (kṣaṇa = moment).
According to this theory, the criterion for the existence of a thing is its capacity to produce (kāritva) a useful effect (artha-kriyā). From this criterion of existence, it may be deduced that a thing having existence must be momentary. If, for example, an object like a seed be not accepted to be momentary, but thought to be lasting for more than one moment, then we have to show that it is capable of producing an effect during the each moment it exists. Since it does not, we conclude that it exists only for a moment.