Rationalism
The very essence of rationalism is that it assumes that the reason is the highest faculty in man and the lord of all the rest.
-- Lyman Abbott
The very essence of rationalism is that it assumes that the reason is the highest faculty in man and the lord of all the rest.
-- Lyman Abbott
Introduction
Rationalism, available in the Renaissance Era, is one of the most focused branches, being entirely devoted to science (with slight benefit to happiness and gold). Only two policies in the branch does not directly increase the production of beakers. The branch is mutually exclusive with Piety. Rationalism is a key branch for both science and diplomatic victories, and the left side can be useful for achieving a tech advantage easing domination victories.The Policies
When and why should you open this branch?
The Policies
Rationalism
Adopting Rationalism boosts +15%
Science while the empire is 
Happy. May purchase Great Scientists with 
Faith. This branch cannot be active at the same time as Piety.The opener is straightforward: just ensure that your empire is happy so you could get a nice science boost. The percentage bonus from Rationalism opener is a global modifier, so it even multiplies the percentage bonus from science buildings, making Rationalism appealing to any civilizations that desire fast research speed.
Faith-buying Great Scientists, which could be done after Industrial, further increases the science emphasis of Rationalism, making it the key branch for Science Victories.
Humanism
+1 Happiness from every University, Observatory and Public School.The one of the two policies in the branch that does not directly increase beaker output (Humanism and Sovereignty), Humanism can provide 2
per developed city. If on the cusp of unhappiness, this can be a useful boost and ensures the 15% boost in opener. However, it's generally preferable to go down the left side of the tree first.
Sovereignty
+1
Gold from Science buildings.Requires: Humanism
In practice, Sovereignty is a bit meh and grants little boost, just defraying some of the maintenance costs of your science buildings. This provides between 2-4 extra
per city. Not a great policy choice, but it gives prioritizes to construct science buildings that match the main idea in Rationalism branch. Generally, Sovereignty is the last choice in this policy branch.
Secularism
+2 Science from every Specialist.This will provide 5 science per scientist specialist and cities that can support more specialists will get more benefit. When combined with the right side of the freedom branch and the Statue of Liberty, specialists can become significantly better than working regular tiles. Each scientist specialist, for example, will be providing 1
1 
5 0.5 . With Korean UA, each specialist will provide two additional science. Though all specialists become more useful, micromanagement to avoid popping unwanted great people is advised.
Free Thought
+1 Science from every Trading Post and +17% Science from Universities.Requires: Secularism
The boost to university is neat, making each University providing a 50% bonus to research. The boost to Trading Post output makes trading post covered jungles especially attractive tiles (with Economics, river, Commerce finisher), providing 2
4 3 . The boost can be especially significant to the science output of a puppet empire. However, this policy doesn't work well with Secularism, which emphasizes working specialists.
Scientific Revolution
Increase Science gained from research agreements by 50%. Requires: Free Thought
This boost stacks additively with that from the Porcelain Tower. Scientific Revolution further encourages you to invest on science by signing research agreements and push you forward towards Science Victory.
Finisher
Adopting all Policies in the Rationalism tree will grant 2 free Technologies.Here comes the highlight of Rationalism. The trick is to use it at the best possible time for maximum effect, optimally late game techs that have huge base cost. For example, an immediate push by getting free Replaceable Parts and Plastics can allow Riflemen to instantly become Infantries and sweep the opposition. For science and diplomatic victories, this policy is optimally used in combination with research agreements and scientist bulbing to get all the final techs allowing for victory.
When and why should you open this branch?
Rationalism is good for the following victory conditions:- Science
Unsurprisingly, this branch devoted entirely to science is useful for science victories. The general boost to science, faith-buying Great Scientists, better results from Research Agreements and 2 free techs from finisher all make this branch extremely attractive.
Diplomatic
Many of the techs required for a diplomatic victory are the same as a science victory, so the same teching strategy applies with the goal of getting to Globalization early. The main difference is that you can be broke to get a science victory, while a diplomatic victory requires gobs of money in addition to science.
Domination
Being able to research faster allows you to gain significant tech and unit advantages over the AI. In some cases, it can be as dramatic as Tanks vs. Crossbows. Someday they might figure out that shooting pointy sticks at that scary-looking iron beast isn't a good way to kill it. Until then, the world is yours.
Piety vs Rationalism
These two branches are mutually exclusive; switching from one to the other causes the empire to go into anarchy and negates the effects of the other. Thus generally only one should be chosen and stuck with for the remainder of the game. Piety is almost a must-have for culture victories and for those who desire higher faith. If the extra culture and faith is unneeded, rationalism is generally a better overall branch, especially for ones who are pursuing domination or science victories.Note: Amended from vanilla Rationalism by insaneweasel, this article is in G&K version.