1. Lord Neuberger in Actavis v Eli Lilly [2017] RPC 21, para. 57.
“…The notion of a product or process which infringes despite an immaterial variation from the invention as claimed is by no means new to domestic patent law. Thus, in Walton v Potter & Horsfall (1843) 1 Web PC 585, Tindal C.J. told the jury that they had to decide whether the defendant’s product was “perfectly distinct” from the patented product, or whether it varied “only in certain circumstances, which are not material to the principle and substance of the invention”.

2. Section 5(5) of the Patents, Designs and Trade Marks Act 1883
“A specification, whether provisional or complete, must commence with the title, and in the case of a complete specification must end with a distinct statement of the invention claimed. ”

3. Lord Herschel in Nobel's Explosives Co v Anderson (1895)
“Where a Patentee claims … the combination of two chemical substances named and specified, he cannot extend, or claim to extend, the protection of his patent to a combination of other substances than those which he has so specified, because such other combination may be found to produce a substance having substantially the same qualities and capable of employment in the same way.”

4. Lord Russell in EMI v Lissen (1938)
“The function of the claims is to define clearly and with precision the monopoly claimed, so that others may know the exact boundaries of the area within which they will be trespassers. Their primary object is to limit, and not to extend, the monopoly. What is not claimed is disclaimed. The claims must undoubtedly be read as part of the entire document, and not as a separate document. Nevertheless, the forbidden field must be found in the language of the claims, and not elsewhere.” 

5. Claims EUROPEAN PATENT CONVENTION Article 84
The claims shall define the matter for which protection is sought. They shall be clear and concise and be supported by the description. 

6. Article 69 Extent of protection EUROPEAN PATENT CONVENTION Article 69
The extent of the protection conferred by a European patent or a European patent application shall be determined by the claims. Nevertheless, the description and drawings shall be used to interpret the claims. 

7. Claims EUROPEAN PATENT CONVENTION Article 84
The claims shall define the matter for which protection is sought. They shall be clear and concise and be supported by the description. 

8. EUROPEAN PATENT CONVENTION
Protocol on the Interpretation of article 69
Article 1 General principles
Article 69 should not be interpreted as meaning that the extent of the protection conferred by a European patent is to be understood as that defined by the strict, literal meaning of the wording used in the claims, the description and drawings being employed only for the purpose of resolving an ambiguity found in the claims. Nor should it be taken to mean that the claims serve only as a guideline and that the actual protection conferred may extend to what, from a consideration of the description and drawings by a person skilled in the art, the patent proprietor has contemplated. On the contrary, it is to be interpreted as defining a position between these extremes which combines a fair protection for the patent proprietor with a reasonable degree of legal certainty for third parties.
Article 2 Equivalents
For the purpose of determining the extent of protection conferred by a European patent, due account shall be taken of any element which is equivalent to an element specified in the claims.

9. Lord Neuberger in Actavis v Eli Lilly [2017] RPC 21, para. 56.
As Sir Hugh Laddie wrote … “[t]he Protocol is not concerned with the rules of construction of claims” but with “determining the scope of protection”.

10. Lord Neuberger in Actavis v Eli Lilly [2017] RPC 21, para. 53.
In the “Epilady” patent, where it could not seriously have been suggested that, as a matter of language, a slotted rubber rod falls within the expression “helical metal spring”, even if one was construing those words in the context of the claim in the patent in suit. But, if one departs from ordinary language, it is necessary to have some guidance or to draw some lines, as Lord Hoffmann implied in Kirin-Amgen [2005] R.P.C. 9, at [37]. That is why he promulgated his three questions in Improver [1990] F.S.R. 181, at p By means of an extended version of the ordinary concept of “construction” or “interpretation”, Hoffmann J. explained how our domestic law, as laid down in Catnic [1982] R.P.C.183, implements art.2 of the Protocol and thus, as I see it, how it gives effect to the doctrine of equivalents.

11. The old rules on purposive construction of the claims
Improver Corp v Remington Consumer Products Ltd [1990] F.S.R. 181
The old rules on purposive construction of the claims
“If the issue was whether a feature embodied in an alleged infringement which fell outside the primary, literal or acontextual meaning of a descriptive word or phrase in the claim (‘a variant’) was nevertheless within its language as properly interpreted, the court should ask itself the following three questions:
(1) Does the variant have a material effect upon the way the invention works? If yes, the variant is outside the claim. If no -
(2) Would this (ie that the variant had no material effect) have been obvious at the date of publication of the patent to a reader skilled in the art? If no, the variant is outside the claim. If yes -
(3) Would the reader skilled in the art nevertheless have understood from the language of the claim that the patentee intended that strict compliance with the primary meaning was an essential requirement of the invention? If yes, the variant is outside the claim.

12. Lord Neuberger in Actavis v Eli Lilly [2017] RPC 21, para. 66.
The new test for protection of equivalents:
“(i) Notwithstanding that it is not within the literal meaning of the relevant claim(s) of the patent, does the variant achieve substantially the same result in substantially the same way as the invention, i.e. the inventive
concept revealed by the patent?
(ii) Would it be obvious to the person skilled in the art, reading the patent at the priority date, but knowing that the variant achieves substantially the same result as the invention, that it does so in substantially the same way as the
invention?
(iii) Would such a reader of the patent have concluded that the patentee nonetheless intended that strict compliance with the literal meaning of the relevant claim(s) of the patent was an essential requirement of the invention?”

13. Arnold J in Actavis v Eli Lilly [2015] RPC 21, para. 104.
 Experience shows that patentees resort to arguments about equivalents in three main classes of case. The first is where, with the benefit of hindsight, it can be seen that the patent was unfortunately drafted, whether because of poor instructions from the inventor or poor drafting by his patent attorney or a combination of these things. Improver  might perhaps be regarded as an example of this. The second class is where technology has moved on since the priority or filing date of the patent. Kirin-Amgen  might perhaps be regarded as an example of this. The third class is where the patentee now regrets a decision taken during the course of prosecution of the patent application, whether by himself or by the examiner, and is trying to avoid the consequences of that decision.

14. Kirin-Amgen, paragraph 47.
The Protocol says that the object is to combine a fair protection for the patentee with a reasonable degree of certainty for third parties. How is this to be achieved? The claims must be construed in a way which attempts, so far as is possible in an imperfect world, not to disappoint the reasonable expectations of either side. What principle of interpretation would give fair protection to the patentee? Surely, a principle which would give him the full extent of the monopoly which the person skilled in the art would think he was intending to claim. And what principle would provide a reasonable degree of protection for third parties? Surely again, a principle which would not give the patentee more than the full extent of the monopoly which the person skilled in the art would think that he was intending to claim.