Machiavelli had read Tacitus for instruction on forms of government, republican as well as autocratic, but after his books were placed on the "Index Librorum Prohibitorum", writers on political philosophy (the so-called "black Tacitists"—see above) frequently used the Roman as a stand-in for the Florentine, and the Emperor Tiberius as a mask for Machiavelli's model of a prince. So, writers like Francesco Guicciardini considered Tacitus' work to be an instruction on how to build a despotic state. Following that line of thought (Catholics in appearance reading "Tacitus" instead of Machiavelli's still forbidden "Prince"), the thinkers of the Counter-Reformation and the age of absolute monarchies used his works as a set of rules and principles for political action.