When I discover music by other composers that have formulated an idea that I also have had, two distinct feelings come up.
The first one is that of disappointment. Even if both of us have had the same idea independently (in place and time), it feels disappointing to discover that somebody else might be attributed with having that particular thought and in a way spoil it for oneself.
But the second feeling is a very different, and humbling one. The feeling of being part of something bigger, where certain mind-sets will cause certain ideas to spring.
So when I discovered the Book of Disquiet, I was shocked and deeply moved. To see Pessoa formulate and grasp some of these abstract, impalpable thoughts (that sometimes come to mind but don't seem to be part of anything bigger or important) made me realize that throughout time and space, people always must have had similar ideas and feelings and perceived the world in ways, not so very different from oneself. So reading this book makes you effectively realize that you're being part of humanity.