The presumption in law is in favour of a person's sanity, even though he may be deaf, dumb, or blind.

The terms 'insanity,' 'lunacy,' 'unsoundness of mind,' 'mental derangement,' 'madness,' and 'mental alienation or aberration,' are indifferently applied to those states of disordered mind in which the person loses the power of regulating his actions and conduct according to the ordinary rules of society. The reasoning power is lost or perverted, and he is no longer fitted to discharge those duties which his social position demands. In some cases of insanity, as in confirmed idiocy, there is no evidence of the exercise of the intellectual faculties. It is probable that no standard of sanity as fixed by nature can be said to exist. The medical witness should decline to commit himself to any definition of insanity. There is no practical advantage in attempting to classify the different forms of insanity.

According to English law, madness absolves from all guilt, but in order to excuse from punishment on this ground it must be proved that the individual was not capable of distinguishing right from wrong in relation to the particular act of which he is accused, and that he did not know at the time of committing the crime that the offence was against the laws of God and nature.

Lunatics are competent witnesses in relation to testimony, as in relation to crime, if they understand the nature of an oath and the character of the proceedings in which they are engaged. The judge, as in the case of children, examines the lunatic tendered as a witness as to his knowledge of the nature and obligation of an oath, and, if satisfied, he allows him to be sworn.

A person, if suffering from such a state of mental unsoundness as to be unable to take care of his property, may be placed under the care of the Court of Chancery. The Court then administers his property, and otherwise allows him entire freedom of action.

With regard to the care of lunatics, no person is allowed to receive more than one lunatic into his house unless such house is licensed and the proper certificates have been signed. One patient may be taken without the house being licensed, but the usual certificates must in all cases be signed, and the Lunacy Commissioners communicated with. If a person receives another not of unsound mind into his house, and such person becomes subsequently insane, the person so keeping him renders himself liable to heavy penalties, unless the legal certificates are at once procured and the Commissioners of Lunacy communicated with.

At common law it appears that a lunatic cannot be placed in an asylum unless dangerous to himself or to others, but under the Lunacy Acts the placing of a madman in an asylum is considered as a part of the treatment with a view to the cure of the patient.