"But for" D's conduct, P would not have been injured. If multiple defendants: (1) merged cause scenario- 2 negligent acts combine to harm P- if one/both acts contributed to the injury in a substantial way then one/both D's are jointly liable, or (2) unascertainable cause scenario- 2 negligent acts but only ONE actually caused the harm, burden shifts to both D's to prove their act was not the cause, if neither can prove it, both are jointly liable.