Analogy plays a basic part in engineering which is subject to certain limitations, due to the following reasons:— lack of accuracy of the measuring instruments and of computing data; — simplifying assumptions necessary for establishing the mathematic model; — restrictions due to compulsory computation means and time. The basic criterion of engineering knowledge is efficiency. The engineer works thus on successive approximations, each of them corresponding to a more or less appropriate model of the final result. In order to avoid (...) disappointment, it is very important for him to be conscious of the relativity of his knowledge and of the scope of teh successive approximations. This procedure is the same for other fields of knowledge and provides an opportunity to specify the concept of rigour applying to each. It shows the practical, limited and revisable character of any knowledge not springing from metaphysics. (shrink)