In the 19th Edition of Clerk & Lindsell on Torts, the authors use these words under the heading of Duty of Care:
"... a duty not to inflict damage carelessly."
In Grant, Justice Wright noted negligence is not a strict liability tort: it requires evidence of a a duty of care for it to be actionable:
"It is essential in English law that the duty should be established. The mere fact that a man is injured by another's act gives in itself no cause of action....
"If the act involves a lack of due care, again no case of actionale negligence will arise unless the duty to be careful exists."
In O'Hara, Justice Moran of the Supreme Court of Illinois defined a duty (of care) as:
"... an obligation to conform to a certain standard of conduct for the protection of another against an unreasonable risk of harm."
In the context of tort law generally, and the law of negligence specifically, Justice Ratliff of the Court of Appeals of Indiana opined, in Lucas v Dorsay:
"In order to prevail in a negligence action, the plaintiff must establish that the defendant breached a duty owed to him which proximately caused the injury.
"A duty of care exists when one party assumes such a duty, either gratuitously or voluntarily. The assumption of a duty creates a special relationship between the parties and a corresponding duty to act as a reasonably prudent person."
The duty of care is dynamic and can adapt to the circumstances. For example, this description of the duty of care of a medical doctor by Justice Murphy of the Prince Edward Island Court of Appeal in Harris v Beck, at ¶65:
"Duty of Care – duty to disclose the risks and benefits of the proposed treatment, and any material or special risks associated with the treatment to be determined in relation to the circumstances of each case."
Duty of care is also an essential principle of the law of corporations as officers and directors are often held to a duty of care in regards to their administration and management of the corporations's affairs. Thus, in Re Ticketplanet.com, Justice Gropper wrote:
"The fiduciary duties of directors of a corporation include the duty of care and the duty of loyalty. The duty of care refers to the responsibility of a corporate fiduciary to exercise, in the performance of his or her tasks, the care that a reasonably prudent person would use under similar circumstances. The second prong, the duty of loyalty, derives from the prohibition against self-dealing that inheres in the fiduciary relationship."
- Dugdale, A., and others, Clerk & Lindsell on Torts, 19th Ed. (London: Sweet & Maxwell, 2006), page 383.
- Grant v Australian Knitting Mills,  A.C. 85 at p. 103
- Harris v Beck, 2009 PECA 8
- In re Ticketplanet.com, 313 B.R. 46 (United States Bankruptcy Court, New York, 2004)
- Lucas v. Dorsey Corp., 609 N.E. 2d 1191 (1993)
- O'Hara v. Holy Cross Hospital, 561 N.E. 2d 18 (1990)