In this paper, the first in a series of two, the theories of arching in symmetrical hoppers are discussed in terms of the underlying assumptions related to flow properties of bulk materials. In particular, the link between the form of the instantaneous and effective yield conditions, and the form of the flow function is investigated in detail, and the resulting implications in the arching theories are critically assessed. It is demonstrated that approximations involved in flow properties determination may contradict the assumptions in the arching theory used. The analysis is supplemented with results of direct shear tests on limestone, gypsum, coal, cement, and taconite, which are subsequently used to illustrate the required approximations. Also, some modifications to the existing arching theories are discussed. In a companion paper, the results of this paper are used to predict the value of outlet size critical for arching, which is next compared with results of mid-scale model tests on plane (wedge-type) and conical hoppers.