Under the common law of torts, in order to establish causation, a plaintiff must prove that an actor's conduct "is a substantial factor in bringing about the harm." 2 Restatement (Second) of Torts § 431(a) at 428 (1965). That standard has two dimensions.
First, the evidence must create an inference that the harm would not have occurred in the absence of the actor's conduct. Id. § 431 cmt. a at 429. That showing is commonly referred to as "but-for causation."
Second, the effect of the actor's conduct must not be so insignificant as to make it unreasonable to treat the actor as responsible for the harm. Ibid. The effect of the actor's conduct must be "substantial," rather than "negligible" in producing the harm. Id. § 431 cmt. b at 429. That showing is commonly referred to as "proximate causation."
Thus, both but-for and proximate cause are required to establish causation.