  Many mathematical models utilize limit processes. Continuous functions and
the calculus, differential equations and topology, all are based on limits and
continuity. However, when we perform measurements and computations, we can
achieve only approximate results. In some cases, this discrepancy between
theoretical schemes and practical actions changes drastically outcomes of a
research and decision-making resulting in uncertainty of knowledge. In the
paper, a mathematical approach to such kind of uncertainty, which emerges in
computation and measurement, is suggested on the base of the concept of a fuzzy
limit. A mathematical technique is developed for differential models with
uncertainty. To take into account the intrinsic uncertainty of a model, it is
suggested to use fuzzy derivatives instead of conventional derivatives of
functions in this model.
