 Medieval authors sometimes raised this objection. As a result, we find two kinds of restriction-theories in the medieval literature: (a) general or strong theories that rule out self-reference, and perhaps other patterns of reference too, in innocuous as well as pathological cases; and (b) more specialized or weaker theories that rule out certain forms of reference only when they result in paradox. Walter Burley and William of Ockham, for example, held the latter form of restriction (Spade 1974).