A will may be void, in whole or in part, for many reasons, which may be divided into two great classes, those arising from external circumstances and those arising from the will itself. The main examples of the former class are revocation by burning, tearing, etc., by a later will, or by marriage of the testator (except as below), incapacity of the testator from insanity, infancy or legal disability (such as being a convict), undue influence and fraud, any one of which is ground for the court to refuse or revoke probate of a will. A will being ambulatory is always revocable, unless in one or two exceptional instances. Undue influence is a ground upon which frequent attempts are made to set aside wills. Its nature is well explained in a judgment of Lord Penzance's: "Pressure of whatever character, whether acting on the fears or the hopes, if so exerted as to overpower the volition without convincing the judgment, is a species of restraint under which no valid will can be made." There is nothing corresponding to the "querela inofficiosi testamenti", but unnatural provisions may be evidence of mental defect.