I do believe it is important to investigate a person's statements and views over time for consistency. Such a study can help us understand not only what they thought, but how sincere they may have been.

We should not expect to find absolute, face-value consistency throughout a political career since the details, context and purposes behind an individual's statements change. For instance, I just finished conducting a lengthy study of a Civil War senator who has been almost universally criticized by writers and historians for being a demagogue and opportunist. On the surface, he certainly did appear to change positions as quickly as the popular wind turned course. If you compared his comments from 1855 to comments from 1862, he seemed a completely different man. Yet, when I created a timeline, carefully tracking his speeches, letters, and other recorded views, I found that his "changes" were not so abrupt or selfish as previously believed. Instead, he---like others during the very politically controversial 1850s---adapted his views in response to new problems and events. The main question I asked during my study was whether or not his main principles changed. I found that they did not, despite the adaptations and positional adjustments he made.

The same approach should be taken with Lincoln. His speech as a freshman congressman in 1848 during the Mexican War is important, but cannot be taken at absolute face value in regards to Southern secession. The natural right of revolution is not the same thing as legal secession. Furthermore, his understanding of and approach to the Mexican/Texas situation as a congressman was much different than his position as a newly elected chief executive during a national crisis.