All living organisms respond to stimuli, grow and change, reproduce to create offspring, maintain a stable body temperature, metabolize energy, consist of one or more cells, and pass on their individual genetic traits. For something to be classified scientifically as a living creature, it must possess all of these characteristics.
The Greek philosopher Aristotle is considered to have made the first attempt at creating a system to classify living organisms. He began by labeling an organism as either a plant or an animal, depending on the organism's ability to move. He then distinguished "animals with blood" from "animals without blood," or what today are considered vertebrate animals from invertebrate animals. Under the heading of blooded animals, Aristotle classified organisms as either mammals, reptiles and amphibians, birds, fishes or whales. Under the heading of bloodless animals, he classified organisms as either cephalopods, crustaceans, insects, animals with shells or zoophytes. Aristotle's early work influenced Swedish botanist Carolus Linnaeus to develop his 18th-century concepts of binomial nomenclature and taxonomy to further identify and distinguish shared traits of living organisms. As scientific discoveries of the 19th and 20th centuries have led to better understanding cells and cell function, the studies of cell biology and microbiology have further developed and established the definitive characteristics of living organisms.