Three general questions of climate theory are addressed. First, it is shown that climate sensitivity cannot be determined directly from available data, and hence its computation depends on model assumptions. Second, the relationship between energy-balance models (EBMs), oscillatory climate models, and ice-sheet models (ISMs) is discussed, and it is shown how EBMs can be obtained as a singular limit of certain oscillatory models. Finally, the physical and mathematical aspects of the oscillations are discussed in further detail and related to the stability question for EBMs and for ISMs.