# 2's answer seems especially relevant to your question. I'll try to give it a go as well. Here is the quotation you cited:
"The spirit of rationality that had been so important to the scientific revolution of the 17th century continued to manifest itself in the economic life of the 18th century.". What is the spirit of rationality? How is it manifested in the 18th century in Europe?
Here's the basic meaning of this quotation: During the 1600s, reason -- the disciplined, logical use of the mind to discover empirical truth -- was especially important to scientists. In fact, in the 1600s reason had become so important that a "scientific revolution" occurred. In other words, people began to use rational, intellectually disciplined ways to study the physical universe in ways (and to an extent) that had not been true in earlier centuries. By the 1700s, rational, disciplined, logical, methodical thinking had also become more and more important in economic life. Large businesses were beginning to arise; international trade ballooned; cities became larger and larger; companies began to employ more and more people; etc. For all these reasons, it became especially important to use systematic, disciplined, reliable, proven methods of conducting economic activities.