In the arena of bioethics we can find the ideas trying to deny the right to freedom of conscience to physicians. This is an attack upon morality itself. It is therefore urgent to outline the basic knowledge dealing with conscience and to be aware of the fact that violation of conscience leads to breakdown of conscience. The present article deals with Socrates daimonion and his conviction of objective truth as well as with other similar thoughts of great philosophers and theologians in this context: everybody is able to recognize good and evil, some basic knowledge of the common truth exists in the heart of everyone. The feeling of guilt and the capacity to recognize guilt should not be suppressed: the silence of conscience is the greatest tragedy of man. It is quite clear, that the doctor should not agree with the patients every wish, no matter how destructive it is. His own conscience and personal integrity as well as the fact, that the doctor should be the person of integrity, is a more fundamental notion than the respect to autonomy of the patient. Being engaged in killing [euthanasia] serves as an example of bewildering and silencing of conscience. The right to and the importance of physicians freedom of conscience are emphasized in many international documents; the relevant quotations are presented in the text. Key words: conscience, truth, euthanasia, autonomy, conscientious objection.