Why is my hard drive on /dev

So I'm working through this book called "Linux Basics for Hackers" and he (the author) said that mounting is simply attaching a disk or drive to the filesystem, so it becomes accessible to the kernel. He also said that every attached device to the filesystem is represented by a file in the /dev dir. When I went to /dev I saw sda, sda1, sda2, etc, and I wondered: If the filesystem is on my hard drive, how would the hard drive be attached to the filesystem???