"Q. With this disease, everything he saw, felt and experienced in his mind, he was experiencing with a diseased mind, was he not? A. That's true, yes. Q. So, it must have played some part in his mental process when he killed Roy Luckhee, must it not? A. I think that's fair if you follow that argument. Some part. Q. Some part. The difference between you and Dr Hamilton is how much? A. That's right, yes. Q. And it comes down to this question, whether it is substantial or not? A. Yes, that's correct. Q. But you agree with me, do you not, that the part that it played was more than trivial? A. More than trivial."
"Where a person kills or is a party to the killing of another, he shall not be convicted of murder if he was suffering from such abnormality of mind (whether arising from a condition of arrested or retarded development of mind or any inherent causes or induced by disease or injury) as substantially impaired his mental responsibility for his acts and omissions in doing or being a party to the killing."
"'Substantially impaired' means just that. You must conclude that his abnormality of mind was a [our emphasis] real cause of the defendant's conduct. The defendant need not prove that his condition was the sole cause of it, but he must show that it was more than a merely trivial one, which did not make any real or appreciable difference to his ability to control himself." 16. "
"In legal terms can you advise the difference between trivial and substantial. _____________ The defence say that 'of substance' = substantial. Is this correct?" 18. "
".... was the abnormality such as substantially impaired his mental responsibility for his acts in doing or being a party to the killing? This is a question of degree and essentially one for the jury. Medical evidence is, of course, relevant, but the question involves a decision not merely whether there was some impairment of the mental responsibility of the accused for his acts, but whether such impairment can properly be called 'substantial', a matter upon which juries may quite legitimately differ from doctors."