What is life? What is a living being (jīvātma)?

A living being (jīvātman, sentient being or conscious entity) is defined as that which has life. What is life? Oxford dictionary defines:

[mass noun] The condition that distinguishes animals and plants from inorganic matter, including the capacity for growth, reproduction, functional activity, and continual change preceding death.

And when you look up “animal”:

[noun] A living organism that feeds on organic matter, typically having specialized sense organs and nervous system and able to respond rapidly to stimuli.

And “organic”:

[adjective] Relating to or derived from living matter.

Basically it’s a circular definition: “Life” is that which characterises animals and plants. And such organisms are said to be having life/“living”.

The Oxford definition gets close but the Vedic definition is more precise and comprehensive.

Jīvātman: A jīvātman is an entity that does four actions and undergoes four incidents.

The four actions are:

Eating (āhāra)
Sleeping (nidrā)
Defending out of fear (bhaya)
Reproducing offspring (maithuna)

āhāra-nidrā-bhaya-maithunaṃ ca sāmānyam ētat paṡubhiḥ narāṇām
dharmō hi tēṣāṃ adhikō viṡēṣaḥ dharmēṇa hīnāḥ paṡubhiḥ samānāḥ

It is described in many places, say Hitōpadēṡa 25, Chanakya Niti 17.17, etc.

The four incidents are:

a. Having birth (janma)
b. Having death (mṛtyu)
c. Getting old (jarā)
d. Having disease (vyādhi)

For reference, see Bhagavad Gita 13.9, Vishnu Sahasranama phalaṡruti (janma-mṛtyu-jarā-vyādhi-bhayaṃ naivōpajāyatē).

Every cell is a jīvātman

A comprehensive definition implies that it should cover all corner cases, without exceptions.

The beauty of the above definition is that it holds true for the smallest living being (single-celled organism like amoeba) to the largest living beings (elephant and whale).

A single-celled microbe certainly consumes food and reproduces more cells of its own type (points 1 and 4 above). Without reproduction, multi-cellular organisms cannot evolve out of it. However, do individual cells sleep (point 2)? Neuroscience says yes

Indeed, even single-cell organisms exhibit circadian-like cycles that dictate a “wake” and a “sleep” state.

How about defending oneself out of fear (point 3)? We would all be dead if the cells of our immune systems didn’t fear and defend our bodies! Science says that cells do fight and defend each other..

And scientists do distinguish between living cells and dead cells (points a. and b. above). Cellular aging is also well-known, meaning, cells do get “old” (point c.). And I don’t need to explain cellular disease – see for example, mitochondrial diseases (point d.).

What does this all mean? Every cell is a jīvātman. So the Divine resides in its “heart” too. And our human body, being made up of millions of cells, is full of life too. Not just the “I” who is experiencing life but countless other cells on our skin, heart and brain.

And Hinduism is not alone in this. Many scientists believe that individual cells are conscious beings:

It is therefore proposed that a bound conscious experience is a property of an individual cell, not a group of cells. Since it is unlikely that one specific neurone is conscious, it is suggested that every neurone has a version of our consciousness, or at least some form of sentience.

Hence, we have to apply the principles of Dharma and Ahimsa to ceullar life too, otherwise karmic bondage is guaranteed.