WordNet (r) 3.0 (2006):

life principle
    n 1: a hypothetical force to which the functions and qualities
         peculiar to living things are sometimes ascribed [syn:
         vital principle, life principle]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "life principle":
   anima, animating force, astral body, atman, bathmism,
   beating heart, biological clock, biorhythm, blood, breath,
   breath of life, buddhi, divine breath, divine spark, elan vital,
   essence of life, force of life, gross body, growth force, heart,
   heartbeat, heartblood, impulse of life, individual essence,
   inspiriting force, jiva, jivatma, kama, life breath, life cycle,
   life essence, life force, life process, lifeblood, linga sharira,
   living force, manas, mind, ousia, physical body, pneuma, prana,
   principle of desire, seat of life, soul, spark of life, spirit,
   sthula sharira, substance of life, vis vitae, vis vitalis,
   vital energy, vital flame, vital fluid, vital force,
   vital principle, vital spark, vital spirit