What is the error in sentence 10?

Directions for the question:

The following passage is an early draft of an essay. Some parts of the passage need to be rewritten. Read the passage and select the best answer for the question that follows. Keep in mind that some questions are about particular sentences or parts of sentences and ask you to improve sentence structure or word choice. Other questions ask to consider organization and development. In choosing the answer, follow the requirements of standard written English.

[1] The whole progress of human civilization beyond its earliest stages has been made possible by the invention of methods of thought. [2] These methods enable us to interpret and forecast the working of nature more successfully than we could if we merely followed the line of least resistance in the use of our minds. [3] These methods, when applied in politics, still represent a difficult and uncertain art rather than a science producing its effects with mechanical accuracy.
[4] When the great thinkers of Greece laid down rules for valid reasoning, they had the needs of politics particularly in their minds. [5] If Plato could visit us now, he would learn that while our artisans proceed by rigorous and confident processes to exact results, our statesmen, like the artisans of ancient Athens, still trust to empirical maxims and personal skill. [6] Why is it, he would ask us, that valid reasoning has proved to be so much more difficult in politics than in the physical sciences?
[7] The universe which presents itself to our reason is the same as that which presents itself to our feelings and impulses—an unending stream of sensations and memories - every one of which is different from every other. [8] Our first answer might be found in the character of the material with which political reasoning has to deal. [9] Before these, unless we can select and recognize and simplify, we must stand helpless, unable to either act or think. [10] Man has therefore to create entities that shall be the material of his reasoning, just as he creates entities to be the objects of his emotions and the stimulus of his instinctive inferences. [11] Exact reasoning requires exact comparison. [12] In the desert or the forest there were few things which our ancestors could compare exactly. [13] The heavenly bodies seem, indeed, to have been the first objects of consciously exact reasoning, because they were so distant that nothing could be known of them except position and movement, and their position and movement could be exactly compared from night to night.