It happens sometimes that a man who has a habit, finds it difficult to act in accordance with the habit, and consequently feels no pleasure and complacency in the act, on account of some impediment supervening from without: thus a man who has a habit of science, finds it difficult to understand, through being sleepy or unwell.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic science understanding
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17897/pg17897-images.html

Context

“for the virtue which regards the end is the chief and moving principle in respect of those things that are referred to the end. Therefore it is necessary to have the moral virtues together with charity. Reply Obj. 2: It happens sometimes that a man who has a habit, finds it difficult to act in accordance with the habit, and consequently feels no pleasure and complacency in the act, on account of some impediment supervening from without: thus a man who has a habit of science, finds it difficult to understand, through being sleepy or unwell. In like manner sometimes the habits of moral virtue experience difficulty in their works, by reason of certain ordinary dispositions remaining from previous acts. This difficulty does not occur in respect of acquired moral virtue:” source