2b: What Is an Organism?

If you go to a high-school biology textbook, it will probably try to define “organism” by listing a suite of characteristics that all organisms supposedly possess. It is surprisingly difficult, however, to come up with a single list of characteristics which covers everything from bacteria to elephants while excluding viruses and tumors.

Philosophers of science get closer to the heart of the matter when they say that only organisms are “the executives of their own existence” possessing “intrinsic powers for self-development.”¹ This is not to say that external factors are unnecessary for development or survival, but that an organism possesses within itself a coordinated and internally directed drive to use those factors in order to achieve a certain fullness of being, particular to its species. This is what makes a bacterium an organism but not your skin cell; it is what makes an infertile monkey an organism but not a mitochondrion; and it is what makes an embryo an organism but not the egg from which it came.