What is the "neighbour principle" and what is the test for it?

Donoghue v. Stevenson stated a general conception of the relations giving rise to a duty of care in what has become known as the “neighbour principle”: You must take reasonable care to avoid acts or omissions which you can reasonably foresee would be likely to injure your neighbour. Who, then, in law is my neighbour? The answer seems to be — persons who are so closely and directly affected by my act that I ought reasonably to have them in contemplation as being so affected when I am directing my mind to the acts or omissions which are called in question.

The relations upon which a duty of care will be found to exist is not frozen in time and changes to meet the needs of society. Hedley Byrne v. Heller & Partners Ltd., [1963] 2 All E.R. 575 stated: How wide the sphere of the duty of care in negligence is to be laid depends ultimately upon the Courts’ assessment of the demands of society for protection from the carelessness of others.