To answer the fundamental question, “What is life?”, it is essential to understand the difference between the states of being alive and not-alive. Where in the life system should we look in order to define and extract the state of being alive? Compatibility between homeostasis (robustness to noise and environmental changes) and plasticity (dynamic transferability between various states) is a universal characteristic of living organisms, besides their potentiality for growth. How is this compatibility achieved? Advances in experimental techniques have allowed us to quantitatively measure a huge number of state variables in living organisms. By integrating these experimental data and extracting a few macroscopic variables which can be related with biological growth or activities, we seek to establish a theory for biological homeostasis (robustness) and plasticity, which characterizes the dynamic states of living organisms.