Lord Hoffmann said at page 45,
‘The question of whether an invention was obvious had been called “a kind of jury question” (see Jenkins L.J. in Allmanna Svenska Elektriska A/B v. The Burntisland Shipbuilding Co. Ltd. (1952) 69 R.P.C. 63, 70 ) and should be treated with appropriate respect by an appellate court. It is true that in Benmax v. Austin Motor Co. Ltd.  A.C. 3705 this House decided that, while the judge’s findings of primary fact, particularly if founded upon an assessment of the credibility of witnesses, were virtually unassailable, an appellate court would be more ready to differ from the judge’s evaluation of those facts by reference to some legal standard such as negligence or obviousness. In drawing this distinction, however, Viscount Simonds went on to observe, at page 374, that it was “subject only to the weight which should, as a matter of course, be given to the opinion of the learned judge”.
The need for appellate caution in reversing the judge’s evaluation of the facts is based upon much more solid grounds than professional courtesy. It is because specific findings of fact, even by the most meticulous judge, are inherently an incomplete statement of the impression which was made upon him by the primary evidence. His expressed findings are always surrounded by a penumbra of imprecision as to emphasis, relative weight, minor qualification and nuance ( as Renan said, la vérité est dans une nuance ), of which time and language do not permit exact expression, but which may play an important part in the judge’s overall evaluation. It would in my view be wrong to treat Benmax as authorising or requiring an appellate court to undertake a de novo evaluation of the facts in all cases in which no question of the credibility of witnesses is involved. Where the application of a legal standard such as negligence or obviousness involves no question of principle but is simply a matter of degree, an appellate court should be very cautious in differing from the judge’s evaluation.