 Frege continues his criticism of psychological logic in the ‘Foreword’ of his Grundgesetze der Arithmetik (1893). He begins by pointing out that the word ‘law’ is ambiguous: it can refer to descriptive or to prescriptive laws. Examples of the former are the laws of physics, examples of the latter are moral laws (1893, XV). Frege suggests that every descriptive law can be reformulated as a prescription to think in accordance with it. For instance, true descriptive laws ought to be accepted. Hence they can be prefixed by the prescription: ‘accept the truth of …’. In other cases the prescription might be a set of instructions on how to reach the truth stated in a descriptive law. Frege’s main point in all this is that whereas all prescriptive laws can be categorized as ‘laws of thought’ on the basis that they tell what we ought to think, only one kind of descriptive laws deserves to be come under the same heading: the set of psychological descriptive laws (ibid.). 