Be sure to distinguish between them. Principles are basic truths, rules or assumptions. Your legal arguments will often be based upon legal rules, also called doctrines or principles . The words principal can be either a noun or an adjective. As a noun it refers to the individual or party who is first in importance, rank, or degree.
Schools employ principals and vice-principals.
The principal is one who has the prime obligation in a contract, or one who, in criminal law, commits a crime. The discriminating writer maintains the distinction between these two words, which have become a shibboleth for educated usage. Lay , the word more often used, is properly a transitive verb, which means that it must be followed by a noun as object. Thus you lay a book on the table, lay down a law, and lay bricks to make a wall. The problem with lie and lay is that the past tense of lie is identical with the present tense of lay.