Immortality is the ability to live forever, or eternal life. Biological forms have inherent limitations which medical interventions or engineering may or may not be able to overcome. Natural selection has developed potential biological immortality in at least one species, the jellyfish Turritopsis nutricula. Certain scientists, futurists, and philosophers, have theorized about the immortality of the human body, and advocate that human immortality is achievable in the first few decades of the 21st century, while other advocates believe that life extension is a more achievable goal in the short term, with immortality awaiting further research breakthroughs into an indefinite future. Aubrey de Grey, a researcher who has developed a series of biomedical rejuvenation strategies to reverse human aging (called SENS), believes that his proposed plan for ending aging may be implementable in two or three decades. The absence of aging would provide humans with biological immortality, but not invu...