"The evidences by which the mental capacity of persons are determined are so varied and complex, and the degrees of such capacity are so numerous, that no specific or circumstantial definition of the term may be given with any accuracy. For that reason, it is doubtful if a court may properly in any given case lay down any but a general rule for the guidance of a jury to whom the issue is submitted. The general definition of the term “mental capacity” seems to contemplate the ability to understand the nature and effect of the act in which a person is engaged and the business he is transacting. . . . We think the charge requested by appellants, that mere mental weakness alone would not amount to mental incapacity, would have so offended. It is true that mere mental weakness does not of itself incapacitate a person (Beville v. Jones, 74 Tex. 148, 11 S. W. 1128), yet to so instruct the jury in this case would have had the effect of unduly stressing a single element of mental incapacity, and would in a sense have been upon the weight of the testimony, and for these reasons was objectionable. . . . Memory, for instance, is not an essential element of capacity to convey property, nor is it essential that the grantor have a full knowledge and understanding of what his entire estate consists, as the jury might infer from the court's charge here. These elements, it is true, are essential to constitute capacity to make a will, but they are not essential in order to give validity to a conveyance of particular property."