Lincoln had several good arguments against slavery
This was in a party that had a strong disdain for slavery and enjoying wide spread support in the North where the institution was frowned upon. Lincoln use of slavery to drive his political agenda was in full cognizant that it would tear the country into half.
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The first reason is their size. They are very small and are very difficult to see. Special technology needed to be developed in order to observe them and their behavior and reach the conclusion that they were in fact infectious agents that could cause diseases. This wasn't possible before because the microscopes weren't as well developed.
Another is that they have a very long incubation period. This means that if you have them, a long time will pass before they start harming you which meant that many people believed them to be harmless since they couldn't track their behavior over a longer time period, also due to the lack of good technology.
The two main reasons why Douglass opposed John Brown in his plan to raid Harpers ferry is because first he was sure that the plan would fail and thus it would lead to many black deaths. The second reason is that Douglas thought that this plan would hurt the abolition movement due to a backlash.
