Homeopathy is a form of alternative medicine based upon principles first defined by Samuel Hahnemann in 1796. A central thesis of homeopathy is that an ill person can be treated using a substance that can produce, in a healthy person, symptoms similar to those of the illness. Practitioners select treatments according to a patient consultation that explores the physical and psychological state of the patient, both of which are considered important to selecting the remedy. According to Hahnemann, serial dilution, with shaking between each dilution, removes the toxic effects of the substance, while the essential qualities are retained by the diluents (water, sugar, or alcohol).