What is the mental element of a testamentary capacity?

The leading case with respect to the mental element involved in testamentary capacity is Banks v. Goodfellow (supra). As to the power of a person to make a will, Cockburn, C.J., said at page 565: It is essential to the exercise of such a power that a testator shall understand the nature of the act and its effects; shall understand the extent of the property of which he is disposing; shall be able to comprehend and appreciate the claims to which he ought to give effect; and, with a view to the latter object, that no disorder of the mind shall poison his affections, pervert his sense of right, or prevent the exercise of his natural faculties - that no insane delusion shall influence his will in disposing of his property and bring about a disposal of it which, if the mind had been sound, would not have been made. Here, then, we have the measure of the degree of mental power which should be insisted on. If the human instincts and affections, or the moral sense, become perverted by mental disease; if insane suspicion, aversion, take the place of natural affection; if reason and judgment are lost, and the mind becomes a prey to insane delusions calculated to interfere with and disturb its functions, and to lead to a testamentary disposition, due only to their baneful influence - in such a case it is obvious that the condition of the testamentary power fails, and that a will made under such circumstances ought not to stand. But, what if the mind, though possessing sufficient power, undisturbed by frenzy or delusion, to take into account all the considerations necessary to the proper making of a will, should be subject to some delusion, but such delusion neither exercises nor is calculated to exercise any influence on the particular disposition, and a rational and proper will is the result; ought we, in such case, to deny to the testator the capacity to dispose of his property by will?