"Obviousness or "inventive step" as the Act calls it (see above), is dealt with in section 3 of the Act:
The burden of proof is upon the party seeking to impugn a patent on this ground and this is normally taken up by adducing appropriate expert evidence. In this connection, the party attacking a patent will seek to marshal and prove evidence as to the state of the art in the light of which a particular step is alleged to be obvious. An unusual feature of this case, already noted, is that Hoover have adduced no evidence at all as to the common general knowledge of the art, even though they were in an advantageous position (so one would have thought), to have done so. The only relevant evidence of common general knowledge before me is that of Mr Dyson, being his own experience of contemporaneous events and the inferences which the court can properly draw from it. But that is secondary evidence, which as has been said in a number of authorities, is merely an aid in assessing the primary evidence. I must state at the outset therefore that this lacuna has not assisted the court in assessing the likely status and impact of several prior art citations relied on by Hoover on the mind of the skilled man at the priority date. Hoover were criticised by Dyson in this respect and in my judgment, that criticism is justified. Thus is Hoover's part of the case.
As with lack of novelty, the parties were on common ground with regard to the proper approach to obviousness, though each party stressed different aspects of it by reference to relevant authority. I was for example reminded by Dyson of the cautionary remarks of Fletcher Moulton L J in British Westinghouse Electric & Manufacturing Co Ltd -v- Braulik (1910) 27 R.P.C. 209 at 230:
I was also reminded of Lord Diplock's remarks in Technograph (supra, at p.362) where he said:
Hoover drew my attention to the impropriety of attaching weight to the evidence of what the inventor did since (for example) he may or may not be the apotheosis of the hypothetical skilled man. As Laddie J said in Hoechst Celanese Corp -v- BP Chemicals Ltd  F.S.R 547 at 565:
Further on the Hoechst Celanese case, Dyson drew my attention to a proposition upon which they strongly rely on the question of obviousness, that is, that for obviousness to be established, there must be a motive for taking allegedly obvious step. Support for this proposition is to be found both from the words of Laddie J on p. 575 and from the judgment of Slade L J Hallen Co v Brabantia (UK) Ltd  R.P.C 195 at 212 which Laddie J quotes:
Laddie J has integrated the above requirement of "motive" into the well-established approach of the European Patent Office to inventive step, namely, that of enquiring as to whether a particular solution to a problem is or is not an obvious one to take. He cites various authorities and concludes this:
In my judgment, if at the relevant time, the skilled man perceived no problem which required to be solved, the task of the protagonist wishing to establish that the taking of a particular step (or a fortiori, steps) which would bring an item of prior art within the scope of the claim under attack was obvious, becomes considerably more difficult. However, as the next authority makes clear, one must assume that the skilled man will approach the prior art citation with interest; there is no question for instance of spontaneous rejection of matter forming part of the state of the art, as at one stage, Dyson came close to suggesting.
It was common ground that the issue of obviousness should in general be approached in the fourfold structured manner proposed by Oliver L J in Windsurfing International v Tabur Marine (Great Britain) Ltd  R.P.C 59 at 73:
A little earlier the judge observed (p. 72):
Dyson Appliances Ltd -v- Hoover Ltd  Reports of Patent, Design and Trade Mark Cases