The second proposition which it was sought to extract from Caswell's Case [3] is in my view still more erroneous. Seizing upon the sentence in Lord Wright's speech - "The jury have to draw the line where mere thoughtlessness or inadvertence or forgetfulness ceases and where negligence begins" [4] - counsel argued that this meant there was a distinction between thoughtlessness, inadvertence, forgetfulness and negligence. He said that the first three on the one hand, and negligence on the other, were mutually exclusive concepts. There is obviously no objection to describing conduct which briefly falls short of the highest standard of care as inadvertent; and, if it does not amount in all the circumstances to negligence it is not negligence. The two terms were used in that sense not only in Caswell's Case [1] but following it in Cummings v. Richard Thomas & Baldwins Ltd. [2] where Lord Porter said: "I am inclined to agree with the courts below that it was inadvertence rather than negligence on the respondent's part which led to the accident" [3] . But, whereas the law knows what is meant by negligence, it has no precise definition of inadvertence which would make it a distinct legal concept. Counsel offered us no definitions in support of his proposition. And the field is one in which the use of abstract words, imprecise and undefined, descriptive of states of mind, is never helpful and can be misleading. The propositions that we heard would surely be surprising to a student of analytical jurisprudence - to anyone who remembered, for example, Austin's statement that "The states of mind which are styled "Negligence" and "Heedlessness", are precisely alike. In either case the party is inadvertent." (Jurisprudence , 4th ed. (1873) p. 440), or who was acquainted with Hearn's adoption of Austin's categories (Legal Rights and Duties, pp. 104-106), or with Salmond's use of the term "inadvertent negligence" (Jurisprudence , 6th ed. (1913) p. 35), to say nothing of more modern writers and of the terminology of psychology to-day. Moreover, in ordinary parlance, a thoughtless act when a reasonably prudent man would take thought, an inadvertent act when he would be vigilant, an act done in forgetfulness of something that if he were careful he would not overlook, may amount to negligence on his part. What Lord Wright said was that mere thoughtlessness or inadvertence is not necessarily negligence. That is indisputable. The mental processes of the plaintiff and of the defendant in a negligence action, although relevant in so far as they were related to their actual knowledge of the risks of what they were doing, are not decisive of the question whether there was negligence or contributory negligence, for that has to be judged by external and objective standards. The law of negligence is concerned with acts, with conduct that is negligent rather than states of mind. The accompanying mental processes of the individual are circumstances, not the fact in issue. As Tindal C.J. put it, "Instead, therefore, of saying that the liability for negligence should be co-extensive with the judgment of each individual we ought rather to adhere to the rule which requires in all cases a regard to caution such as a man of ordinary prudence would observe": Vaughan v. Menlove [4] . Counsel for the plaintiff asked the learned trial judge at the end of his summing up to direct the jury that "a heedless or inadvertent act is not necessarily negligent". His Honour assented, saying simply: "A heedless or inadvertent act, which was what I attempted to put before, is not negligent. It has to show a lack of reasonable care for his own safety. It is a question for you whether you think a heedless or inadvertent act shows a lack of reasonable care for his own safety". That, taken with the summing up as a whole, could have left the jury under no misapprehension as to the question they had to decide. It certainly was not unfavourable to the plaintiff.