Vitalism is the theory that living things are alive because of some "vital force", some special substance or force or essence that nonliving things lack. Vitalism has been totally discredited by modern science; it is very evident from molecular biology that living things are alive because of a certain organization of nonliving matter. However, many alternative-medicine advocates propose some vitalist theory as a "theoretical justification" of their practices. Theories of qi, prana, orgone, mysterious "energies", etc.
Closely related to vitalism is mind-body dualism, the theory that there is some nonphysical substance or force responsible for mind. In fact, some conceptions of the soul have that entity responsible for both life and mind, thus linking vitalism and dualism. But also like vitalism, dualism has become discredited by modern science.
Vitalism is an ancient and likely universal premodern belief. "Soul" and "spirit" can easily be interpreted as vital force. The ancient Greek atomists, well-known for their philosophical materialism, believed that there are vital-force (soul) atoms as well as other kinds of atoms. Aristotle even went so far as to identify three kinds of vital force: the vegetable soul, the animal soul, and the rational soul.
Though it is widely believed to have become discredited by the 1828 synthesis by Friedrich Wöhler of urea from ammonium cyanate, that appears to be a myth. Anthony Cheng has written on The Real Death of Vitalism : Implications of the Wöhler Myth (Penn Bioethics Journal, vol. 1, iss. 1):
The Wöhler Myth, as historian of science Peter J. Ramberg calls it, originates from one account by Bernard Jaffe, the author of a popular history of chemistry in 1931 that is still in print today. “Ignoring all pretense of historical accuracy, Jaffe turned Wöhler into a crusader who made attempt after attempt to synthesize a natural product that would refute vitalism and lift the veil of ignorance, until ‘one afternoon the miracle happened’”
(Ramberg, Peter J. (2000) The Death of Vitalism and the Birth of Organic Chemistry. Ambix, 47(3),170-195.)
But it was nevertheless counterevidence against a common view at the time, notably advocated by Jöns Jacob Berzelius, that many compounds, the "organic" ones, could only be made by living things. The others are "inorganic". Wöhler's synthesis would likely have remained a curiosity if it had not been followed by many others. But it was. In 1845, one of Wöhler's students, Adolph Kolbe, succeeded in making acetic acid from inorganic compounds, and in the 1850's, Marcellin Berthelot succeeded in synthesizing numerous organic compounds from inorganic precursors, including methyl alcohol, ethyl alcohol, methane, benzene, and acetylene. They and their colleagues also tackled larger molecules, showing that they were composed of smaller ones.
This work thoroughly discredited that particular vitalist theory, though there were many other areas that vitalists could point to. Some of them indeed did, like one of the last reputable vitalists in biology, Hans Driesch. In 1895, he made an odd discovery: he could take a fertilized sea-urchin egg that had started dividing, split it in two, and watch the two halves develop into two complete sea urchins, instead of two halves of one sea urchin. He concluded from this that there was some "vital force" responsible for development. But it was later discovered that in their first few divisions, a sea-urchin embryo's cells are uncommitted to any particular fate. That commitment only happens later, and Driesch had proposed a sort of "vital force of the gaps", something like a God of the gaps. Stem cells are well-known uncommitted or partially-committed cells.
Vitalists could claim that organism metabolism involves vital force, but around then, biologists started discovering counterevidence. Eduard Buchner, discovered in 1897 that yeast-cell contents could cause fermentation in the absence of whole yeast cells. He followed up in 1903 by making the first discovery of one of the enzymes responsible (zymase). His successors then mapped out many metabolic pathways in great detail, including biosynthesis ones.
But not long after Buchner's work, Jacques Loeb published in 1912 a landmark work, The Mechanistic Conception of Life. He described experiments on how, as Bertrand Russell put it (Religion and Science), a sea urchin could have a pin for its father. He also offered this challenge:
... we must either succeed in producing living matter artificially, or we must find the reasons why this is impossible.
(pp. 5-6). It looks as if he was challenging vitalists to show that vitalism is more than some theory of vital force of the gaps. In pp. 14-15, he took another swipe at vitalism:
It is, therefore, unwarranted to continue the statement that in addition to the acceleration of oxidations the beginning of individual life is determined by the entrance of a metaphysical "life principle" into the egg; and that death is determined, aside from the cessation of oxidations, by the departure of this "principle" from the body. In the case of the evaporation of water we are satisfied with the explanation given by the kinetic theory of gases and do not demand that to repeat a well-known jest of Huxley the disappearance of the "aquosity" be also taken into consideration.
Over the twentieth century and continuing to the present day, molecular biologists have discovered numerous molecular-scale mechanisms, like the well-known carrier of heredity DNA and its relative RNA. Even though some problems, like development, continue to be very difficult, biologists have yet to find any trace of vital force.