The symptom of free will is self-determination. Non-living bodies obey mechanical laws of nature and act according to external forces. But the living organisms utilize physical laws in carrying out their biological functions; therefore physical laws are not sufficient to describe them. For e.g., a bird’s flight path cannot be calculated from Newton's laws of motion. Plants grow above the ground against the force of gravity, i.e. they exhibit negative gravitropism. The movements of organism show self-determinism. Mangroves intentionally generate breathing roots against gravity.