On the other hand we should under certain circumstances be ready to substitute “B knows the formula”, “B has said the formula” for “B can continue the series”. As when we ask a doctor, “Can the patient walk?”, we shall sometimes be ready to substitute for this, “Is his leg healed?” – – “Can he speak?” under certain circumstances means, “Is his throat all right?”, under others (e.g. if he is a small child) it means, “Has he learned to speak?” ‒ ‒ To the question, “Can the patient walk?”, the doctor's answer may be, “His leg is all right”. ‒ ‒ We use the phrase, “He can walk, as far as the state of his leg is concerned”, especially when we wish to oppose this condition for his walking to some other condition, say the state of his spine. Here we must beware of thinking that there is in the nature of the case something which we might call a || the complete set of conditions, e.g. for his walking; so that the patient, as it were, must walk || can't help walking if all these conditions are fulfilled.