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We present a generalisation of the Maier-Saupe (MS) theory that incorporates the description of the ferroelectric nematic phase (NF) within its scope of application. The extension of the theory is carried out in a natural way, by adding to the nematic potential of MS a term proportional to cosθ and to the polar order parameter , where θ is the angle between the long molecular axis and the spontaneous polarisation. The NF phase can be reached by cooling from the isotropic phase with or without the formation of an intermediate normal nematic phase, depending on the relative intensity of the polar and non-polar terms of the nematic potential. For both phase sequences all the transitions are first order. The temperature dependence of the polar and non-polar order parameters is calculated, and some results derived from the theory are compared with the experimental observations. It is argued that the theory is a valuable first approximation to describe some of the properties of ferroelectric nematics. [ABSTRACT FROM AUTHOR]