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The use of placebos within controlled clinical trials for the last 50 years suggests that psycho-physiological effects can be objectified and measured. These are related to the therapeutic results covered by the term "placebo effect". Since Beecher's pioneer article in 1955, this effect is recognised and cited in medical articles as being on average 35% effective on the majority of symptoms. However, the meaning of these measurement is questionable. For example, it does not take into account the spontaneous evolution of symptoms and diseases. In fact, the placebo effect is not the effect measured in the group treated by the placebo. The clinical trial drug controlled against placebo does not make it possible to measure what is thus wrongly called "the placebo effect". Is it possible then to measure and to objectify what is called the placebo effect? And if that is possible, how? This article calls into question the extent of the placebo effect evaluated by means of controlled clinical trials, without denying its importance in medical practice. Firstly it raises the epistemological problem of the real possibility, via experimentation, of objectifying psychological therapeutic effects. And secondly, the question is also raised of the interest for medical care of objectifying those effects.