The Pali texts use two terms to refer to the energy, or force, or will, that sustains life. The terms are āyusaṅkhāra and jīvitasaṅkhāra. Normally, these would be synonyms, however it may not be as clear cut as that.
Warning: this is a boring article with a predictable conclusion. I wrote it while doing the research, thinking it might lead somewhere interesting, but it didn’t. Oh, well. Anyway, you can skip to the last paragraph if you want.
In interpreting these it’s important to bear in mind that in ancient times it was commonly held that there was a distinct force or energy, vitality, which sustained life. Whether this is intended by these terms is unclear, but it should be borne in mind. This is philosophically interesting, as it raises the question of the extent to which the Buddha accepted the reality of physical phenomena rejected by modern science.
One of the problems is the wide range of meanings of saṅkhāra. Here it might mean either “will”, in the sense of the “will to live”, or “energy(s)” as in “vital forces”.
The best known context for these is the Mahāparinibbāna Sutta (DN 16, also SN 47.9). There, the Buddha first refers to jīvitasaṅkhāra (singular) when deciding to remain alive for the time being so as to address his followers.
Yannūnāhaṃ imaṃ ābādhaṃ vīriyena paṭipaṇāmetvā jīvitasaṅkhāraṃ adhiṭṭhāya vihareyyan
Why don’t I forcefully suppress this illness, resolve on the jīvitasaṅkhāra and live on?
In this context, I find the use of the term adhiṭṭhāya interesting. Normally it means to “apply to, concentrate on, commit to, stabilize”. It thus seem to fit nicely with the idea that jīvitasaṅkhāra here means “will to live”. We could render it as “commits to the will to live”. Nevertheless, while on the face of it I find this reading the most natural, it is not at all impossible that this means “resolves upon the life force”. In this sense, we might render it “stabilize the life force”.
Later, (found in DN 16 and AN 8.70) he relinquishes the āyusaṅkhāra (also singular):
Atha kho bhagavā cāpāle cetiye sato sampajāno āyusaṅkhāraṃ ossaji
Then at the Capala shrine the Buddha, mindful and aware, relinquished the āyusaṅkhāra.
Since one appears after the other, there is no contradiction if they mean the same thing. The terms appear rarely in other contexts, let us see.
SN 20.6 has:
Yathā ca, bhikkhave, tassa purisassa javo yathā ca candimasūriyānaṃ javo yathā ca yā devatā candimasūriyānaṃ purato dhāvanti tāsaṃ devatānaṃ javo, tato sīghataraṃ āyusaṅkhārā khīyanti.
As fast as that man is, as fast as the sun and moon are, and as fast as the gods that run before the sun and moon are, the waning of the āyusaṅkhāras (plural) is faster.
Here the āyusaṅkhāras are something that fade and fail, whose running out presumably leads to death. A similar sense appears to be the case at Mil 5.3.6:
Na, mahārāja, tatonidānaṃ bhagavato koci anuppanno rogo uppanno, api ca, mahārāja, bhagavato pakatidubbale sarīre khīṇe āyusaṅkhāre uppanno rogo bhiyyo abhivaḍḍhi.
Great King, it’s not for that reason that a new illness afflicted the Buddha. Rather, when the Buddha became naturally weak, with the waning of the āyusaṅkhāra (singular), the illness that was already present flared up.
At Thi-ap 28#5 the sense appears to be the same.
In these cases the sense of “will to live” would not work. It must mean some kind of natural life force or vital energy.
My searching has turned up just one other use of jīvitasaṅkhāra, at Ps 1.6:
Paṭisandhikkhaṇe tayo jīvitasaṅkhārā sahajātapaccayā honti, aññamaññapaccayā honti, nissayapaccayā honti, vippayuttapaccayā honti
At the moment of rebirth-linking the three jīvitasaṅkhāras (plural) are conditioned by conascence …
(Incidentally, this passage is a neat example of why this text, the Patisambhidhamagga, despite appearing in the Khuddaka, must be later than the canonical Abhidhamma texts.)
Despite referring to the three jīvitasaṅkhāras as if it was a recognized term, it doesn’t appear elsewhere as far as I know. The fact that it’s plural while the sutta reference is singular may or may not imply that they refer to different things.
The commentary explains it as the group of three, āyu usmā viññāṇaṃ (life, heat, consciousness), which are found in the suttas at MN 43#51 and SN 22.95#12. They are best known as the energies that distinguish between a meditator in the attainment of cessation and someone who is dead.
If this is accepted, it would seem that jīvitasaṅkhāra is a broader term that includes āyusaṅkhāra. In any case, it cannot mean “will to live”, since it is used precisely to illustrate what happens at death.
So it would seem that all of the cases that yield a clear meaning use saṅkhāra in the sense of “force, energy” rather than “will” here. The sense of “will” is possible only in the sutta source for jīvitasaṅkhāra. This would mean that it differs from the sense in the Patisambhidamagga. This, however, is not a big deal, since that text is several centuries later, and in addition uses a plural form.
In context, I come back to the different sense of the verbs used with these term. While adhiṭṭhāti and ossajjati are not, so far as I know, a recognized pair of opposites, their sense is pretty much opposite: to “stabilize” or “commit to” and to “relinquish”. This suggests that the terms are in sutta usage synonyms.
The association of jīvitasaṅkhāra with adhiṭṭhāti and āyusaṅkhāra with ossajjati is supported by the Sanskrit. There, however, the first context—stabilizing the life force when ill—is missing, and instead we consistently find:
jīvitasaṃskārān adhiṣṭhāyāyuḥsaṃskārān utsṛjati
Having stabilized the jīvitasaṅkhāra he relinquished the āyusaṅkhāra
This is no accident, as we find comparable phrases in multiple texts: the (Mula) Sarvastivada Mahaparinirvana Sutra, the Divyavadana and the Mulasarvastivada Vinaya. I can’t account for the strange sense this yields, unless my Sanskrit is way off (which is quite possible!) My guess would be that the conjunction of terms arose during a confusion in editing of the MPS originally, and became part of the stock lexicon. The simplest reading of these passages, therefore, would be that jīvitasaṅkhāra and āyusaṅkhāra were read as synonyms, and adhiṭṭhāti was taken in a different sense here.
Anyway, the rather unspectacular conclusion of all this is that these terms are probably synonyms, and mean “life force” rather than “will to live”. The differentiation of the two in the Patisambhidamagga is probably a later development.
As far as translation goes, the best term is “vital” as in “vital energies”, etc., since this is the standard term for the idea in western thought. However, these days it’s used mostly in the sense of “necessary”, so it might be confusing. Best stick with “life force” for both.
As a final note, after giving up his “life force” (āyusaṅkhāra) the Buddha speaks some verses, which include the term bhavasaṅkhāramavassaji. Since the verb is the same, and the phrase is referring back to the immediate past events, bhavasaṅkhāra and āyusaṅkhāra must mean the same thing. This is a good example of how bhava often means “life”.