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Ex hypothesi

Latin tag, often used in legal argument, meaning according to the stated hypothesis, or in line with the set of assumptions already made.

By way of example (highlighted in bold): Marks and Spencer plc v BNP Paribas Securities Services Trust Co (Jersey) Ltd [2015] UKSC 72; [2016] AC 742, per Lord Neuberger of Abbotsbury PSC:

“[27] Of course, it is fair to say that the factors to be taken into account on an issue of construction, namely the words used in the contract, the surrounding circumstances known to both parties at the time of the contract, commercial common sense, and the reasonable reader or reasonable parties, are also taken into account on an issue of implication. However, that does not mean that the exercise of implication should be properly classified as part of the exercise of interpretation, let alone that it should be carried out at the same time as interpretation. When one is implying a term or a phrase, one is not construing words, as the words to be implied are ex hypothesi not there to be construed; and to speak of construing the contract as a whole, including the implied terms, is not helpful, not least because it begs the question as to what construction actually means in this context.”