 Underlying this analysis of causal conditioning is the notion of existence as efficacious action, or karma. Karma, a fundamental principle in Buddhist thought from its inception, is what fuels the repetitive experience in saṃsāra, the round of  rebirth.[14]  In Abhidharma exegesis, the efficacious action or distinctive functioning of dharmas is understood predominantly as causal functioning. For the orthodox Sarvāstivāda-Vaibhāṣika, the existence of dharmas as real entities (dravyatas) is determined by both their intrinsic nature and particular causal functioning. Intrinsic nature, however, is an atemporal determinant of real existence. What determines a dharma’s spatio-temporal existence is its distinctive causal functioning: past and future dharmas have capability (sāmarthya) of functioning, while present dharmas also exert a distinctive activity (kāritra). Present activity is an internal causal efficacy that assists in the production of an effect within a dharma’s own consciousness series. It is this activity that determines a dharma’s present existence and defines the limits of the span of its present moment. Capability, by contrast, is a conditioning efficacy externally directed towards another consciousness series: it constitutes a condition that assists another dharma in the production of its own effect (Cox 2004A, 570–573; Williams 1981, 240–243). A dharma’s present activity arises and falls away, but past and future dharmas all have potential for causal functioning and exist as real entities due to their intrinsic nature. For the Sarvāstivāda, this model—which insists on constant change within the limits of the present moment and implies the existence of dharmas in the three time periods—preserves both the principle of impermanence yet explains continuity and causal efficacy.