唯一存在的限制是你给自己设定的限制。
想想看,人类愚蠢到会去小心地避免那些可能使他们造成伤害的臭鼬或是狐狸,却不担心狮子对他们安全造成的威胁。
By used books. They have the same words as the new ones. Also libraries.