Part of speech
本文的内容基于：
What is “part of speech”？
"part of speech"即词性，如果以machine learning的角度来看的话，“part of speech”非常类似于“label”，即它表示的是词语的**类别**（后文中会出现”word class“这个词，显然这个词是更加能够体现它的含义的），这一点在Natural Language Processing with Python的5. Categorizing and Tagging Words的名称中的“Categorizing"（分类）中体现出来了。显然，既然能够进行分类，那么各种part of speech（word class）肯定有着显著的特征。
有了这些认知，就能够理解维基百科的Part of speech中的定义了：
In traditional grammar, a part of speech (abbreviated form: PoS or POS) is a category of words (or, more generally, of lexical items) that have similar grammatical properties. Words that are assigned to the same part of speech generally display similar syntactic behavior—they play similar roles within the grammatical structure of sentences—and sometimes similar morphology in that they undergo inflection for similar properties.
下面给出了part of speech的一个例子：
Commonly listed English parts of speech are noun, verb, adjective, adverb, pronoun, preposition, conjunction, interjection, and sometimes numeral, article, or determiner.
Other terms than part of speech—particularly in modern linguistic classifications, which often make more precise distinctions than the traditional scheme does—include word class, lexical class, and lexical category.
需要注意的是，不同的语言有着不同的part of speech
Because of such variation in the number of categories and their identifying properties, analysis of parts of speech must be done for each individual language.
如何定义（创造）part of speech？
一门语言的可用使用的Part of speech不是固定的，研究人员是可以根据需求来创造适合于特定问题的part of speech（其实就是定义类别，但是显然在进行定义的时候，应该是需要考虑语言学的理论的），这些part of speech称为“Tag sets”。那如何来进行创造呢？下面给出了一些有参考价值的内容：
如何让计算机来自动地进行“分类”？
我们已经了解了part of speech的含义，知道它本质上就是类别。给定一段话，我们人类是可以非常轻松地指出哪些词是属于哪种part of speech，那如何让computer也获得这种能力呢？这就是本节标题的所提出的问题,，这个问题是NLP领域的经典问题，叫做Part-of-speech tagging，在下一章节的Part-of-speech-tagging中对此进行专门讨论。