To aquire a full understanding of human nature in the sense that Seneca has in mind in the quote would be to become wise. The Stoics assumed that it is in fact possible for human beings to reach that goal but they also emphasize that even if we dedicate every moment of our life to reach it our chances are very slim. All we can do is to strive to make progress as every bit of progress will make our life more tolerable - as least according to Seneca (he talks about this in his letter no 16). To make progress we first all have to have a strong desire to make progress and then we have to study human nature by critical reflection on our own experiences and by critical reflection on other peoples thoughts and experiences.

“How much progress can I make?” you say. As much as you attempt. What are you waiting for? No one attains wisdom merely by chance. Money will come of its own accord; public office will be conferred on you; popularity and influence will perhaps be accorded you without any action on your part; virtue, though, will not just happen to you. The work it takes to recognize it is neither easy nor short; but the effort is worth making, for by it one will take possession of every good at once".

- Seneca, Letters 76.5-6

"The mind must be exercised both day and night, for it is nourished by moderate labour and this form of exercise need not be hampered by cold or hot weather, or even by old age. Cultivate that good which improves with the years."

- Seneca, Letters 15.4