“But the likely scope of a duty of care raises a prior rule of law issue. The more broadly a duty of care is framed, the greater the risk that it will stray into impermissible vagueness.
The rule of law objection to vagueness was spelt out by the House of Lords in R v Rimmington, citing the US case of Grayned:
“Vagueness offends several important values … A vague law impermissibly delegates basic policy matters to policemen, judges and juries for resolution on an ad hoc and subjective basis, with the attendant dangers of arbitrary and discriminatory application.“
Whilst most often applied to criminal liability, the objection to vagueness is more fundamental than that. It is a constitutional principle that applies to the law generally. Lord Diplock referred to it in a 1975 civil case (Black-Clawson):
“The acceptance of the rule of law as a constitutional principle requires that a citizen, before committing himself to any course of action, should be able to know in advance what are the legal consequences that will flow from it.”
Certainty is a particular concern with a law that has consequences for individuals’ speech. In the context of a social media duty of care the rule of law requires that users must be able to know with reasonable certainty in advance what of their speech is liable to be the subject of preventive or mitigating action by a platform operator subject to the duty of care.”