At its heart, chiropracty is vitalism. It believes in a life energy and that spine adjustments cure sickness by removing blockages to the flow of that energy. There is no medical or scientific basis for that life energy, and chiropractors typically eschew anything that remotely resembles a controlled scientific study and instead rely on anecdotal evidence.
Physical therapy, when practiced correctly, is based on science and medicine, backed by real academic research. Unfortunately, it is one of the areas of medicine that is also prone to a lot of quackery.
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