Additionally, as understood in the Fullmetal Alchemist universe, humans are comprised alchemically of three major parts: the body, the mind and the soul/spirit.
The physical body, which is a complex system of bones, muscles, organs, nerves and blood vessels developed for optimal interaction with the environment, follows standard natural laws and patterns that, while different between species, are similar to all animal life on the planet. It has been stated that, broken down to its basest components, the average adult human body is comprised of Water (35 L), Carbon (20 kg), Ammonia (4 L), Lime (1.5 kg), Phosphorous (800 g), Salt (250 g), Saltpeter (100 g), Sulfur (80 g), Fluorine (7.5 g), Iron (5 g), Silicon (3 g) and fifteen traces of other elements.
Something interesting to think about is, while the above is indeed what Edward and Alphonse thought was the correct assortment of chemicals, they were actually very wrong. Assuming that the Humans in Fullmental are similar or even equal to humans in real life then the actual list would have been as proceeds; Oxygen, 43kg - Carbon, 16kg - Hydrogen, 7kg - Nitrogen, 1.8kg - Calcium, 1kg - Phosphorus, 0.78kg - Potassium, 0.14kg - Sulfur, 0.14kg - Sodium, 0.10kg - Chlorine, 0.095kg - and Magnesium, 0.019kg. In other words it's very possible that instead of needing a human soul, they simply got the wrong ingredients.
The soul (but often referred to as the spirit), on the other hand, is a component apparently unique to humans in the Fullmetal Alchemist universe. It is the incorporeal and supposedly eternal part of a human, synonymous with the self or psyche, which contains all non-biological and emotional attributes and is said to separate entirely from the physical body at death. The soul also appears to hold a great deal of energy, as it powers human action and can even be used to fuel certain transmutations.
While a human remains alive, however, the body and soul are connected by an entity known as the mind. Regardless of how mutilated or altered either the body or soul may become, so long as they both remain intact, the connection between them remains and will cause the two components to resonate with each other and attract one another in an attempt to reunite themselves.
The will of humans is a concept that is touched upon in the series. As a species with reasoning advanced enough for intimate communication and souls that allow them to identify with each other on a plane other than the purely biological, many humans are able to cooperate, rather than compete, for survival and will even put their own lives at risk to help others.
The Definition of Human
Although the standard human being usually fits a set list of criteria in order to be referred to as "Human", alchemical advancements and human experimentation in alchemy has brought about complications in the cases of those whose bodies or souls have been altered or mutilated.