The second thing to understand in applying the rules is to be familiar with the concept of distribution. It does not say, or imply, that all mammals are cats. The middle term must be distributed at least once. Any term distributed in the conclusion must be distributed in the premises. When applying the rules, it is helpful to first go through the syllogism underlining all distributed terms.
All terriers are dogs.
Therefore, all terriers are mammals. Rule 1 is met in that there are no negative claims in this syllogism. The syllogism is valid. Some Californians are doctors. Therefore, some Californians are not nurses .