二十年前的我和现在的我是同一个人吗？二十年后的那个我呢？为什么？这是我们每一个人都有可能向自己提出的问题，同时也是一个严肃的哲学问题。我们一般称之为个人同一性问题。这是一个古老的问题，它涉及人对自己的定义，对人自身持续存在的渴望以及对人与其身体、心理的关系等问题。同时，这也是一个现代的问题。在当今社会，随着现代医学的迅速进步、人工智能的蓬勃发展，需要人们更清晰地回答“人是什么”、“人与非人的边界在哪里”以及“如何定义人类”等问题。本文认为，对个人同一性的研究有助于以上问题的解决。
当代关于个人同一性问题的讨论，有两个基本的观点，即物理连续性观点和心理连续性观点。个人同一性的物理连续性观点，以舒梅克、威金斯和帕菲特等人为代表，他们认为人的身体、大脑等物理性存在才是判断个人同一性的标准。个人同一性的心理连续性观点，以威廉姆斯、奥尔森、刘易斯和努南等为代表，他们认为人的记忆、个性和心理特征等才是判断个人同一性的标准。然而，自从威金斯提出著名的大脑半球分裂案例后，心理连续性标准遭遇了严重的挑战。对于“分裂”问题，可以简单描述如下：假设布朗先生的两个大脑半球被成功地移植到两个新的健康身体中，接受左半球的人称之为左先生，接受右半球的人称之为右先生，左先生和右先生具有和布朗先生一样的记忆、个性和心理特征；根据常识观点，左先生和右先生明显是两个不同的个体，但根据个人同一性的心理连续性标准，左先生和右先生是同一个，因为他们和布朗先生保持了记忆、个性和心理特征的连续性，问题于是由此产生。面对“分裂”案例的质疑，心理连续性标准的支持者提出了两种解决方案。
分裂问题的第一个解决方案是“无分支”理论，以加勒特、努南等人为代表。该理论认为，在分裂案例中，左先生和右先生如果只有一个幸存，布朗先生才能得以幸存；如果左先生和右先生都幸存，布朗先生将无法幸存。也就是说，只有当延续者只有一个而不存在分支时，我才能保持同一性并幸存下来；一旦有分支幸存下来，则意味着我不存在。很显然，这个观点的结果是，分裂即死亡，这是人们难以接受的。再者，“无分支”理论的支持者更多的是将一个人的延续者没有分支作为个人同一性的前置条件，而无法真正解释为什么会没有分支这个问题。
分裂问题的第二个解决方案是“多重占用理论”，以佩里、刘易斯等人为代表。该理论认为，在分裂案例中，左先生和右先生在分裂之前存在，脑裂手术不过是将他们从空间上分开而已。显然，这种观点会导致一种矛盾，即一个人身上怎么会有两个乃至多个存在者和他同时共存呢？因此，“多重占用理论”通常与一种形而上学主张结合在一起，这种观点认为，人和一般物理对象是“四维物质块”，是由“时间部分”组成的。这种观点通常被称为“四维主义”，以海勒、奥尔森等人为代表。严格说来，“四维主义”应该被称为“时间部分本体论”，因为对该理论的探讨涉及到时间和变化的本质，常常引起许多争论，但它确实对“多重占用理论”提供了更好的解释。
本文认为，目前关于个人同一性问题研究，大多还处在“以人论人”的范围内，试图在人的物理存在和心理存在这两个范围内找到问题的答案。但是，要解决个人同一性问题需要更广泛的理论视域，仅在人自身存在的范围寻找个人同一性问题的答案，很可能是不够的。这个“更广泛的理论视域”，可以从两个方向来描述。第一个方向就是形而上方向，也就是时间，也即是说将个人同一性与形而上学意义上的时间概念紧密结合在一起来进行研究，这个方向上已有的代表性理论就是四维主义。第二个方向可称之为形而下方向，就是将个人同一性研究与对物质存在的研究结合在一起，而这个研究方向当前的代表性理论就是动物主义。本文站在个人同一性与时间概念紧密联系的角度，对“无分支”理论、“多重占用理论”进行了一定程度的批评，指出了这两个理论主张的不足之处在于没有理清它们与时间概念的内在联系，同时也指出四维主义的一些不足。最后，结合对动物主义基本观点的考察，尝试提出一种可称之为“多维主义”的观点，以期调和以四维主义为基础的心理主义和动物主义不一致的地方，为个人同一性问题的解决提供更多的可能。

Am I the same person I was 20 years ago as I am now? What about me 20 years from now? Why? This is a question that each of us may ask ourselves, and it is a serious philosophical question. We call it the problem of personal identity. This is an ancient question, which concerns man's definition of himself, the desire for his continuing existence, and his relationship to his body and mind. At the same time, it is a modern problem. In today's society, with the rapid progress of modern medicine, the vigorous development of artificial intelligence, people need to answer more clearly the questions ‘what is human’, ‘where is the boundary between human and non-human’ and ‘how to define human’. This paper holds that the study of personal identity is helpful to solve the above problems.
There are two basic viewpoints in the contemporary discussion of personal identity, namely, the viewpoint of physical continuity and the viewpoint of psychological continuity. The physical continuity view of personal identity is represented by Shoemaker, Wiggins and Parfit, who believe that physical existence such as human body and brain is the criterion to judge personal identity. The psychological continuity view of personal identity is represented by Williams, Olson, Lewis and Noonan, who believe that people's memory, personality and psychological characteristics are the criteria to judge personal identity. However, since Wiggins' famous case of hemispherical division, the standard of mental continuity has been seriously challenged. The ‘fission’ problem can be simply described as follows: Suppose Mr. Brown’s two brain hemispheres are successfully transplanted into two new healthy bodies, the person who receives the left hemisphere is called Mr. Left, and the person who receives the right hemisphere is called Mr. Right, and Mr. Left and Mr. Right have the same memory, personality and psychological characteristics as Mr. Brown; According to common sense, Mr. Left and Mr. Right are obviously two different individuals, but according to the psychological continuity criterion of personal identity, Mr. Left and Mr. Right are one and the same, because they and Mr. Brown maintain continuity of memory, personality and psychological characteristics, and the problem arises. Faced with the challenge of the ‘fission’ case, proponents of the psychological continuity standard have proposed two solutions.
The first solution to the fission problem was the ‘non-branching’ view, represented by Garrett, Noonan and others. The theory holds that in a split case, if only one of Mr. Left and Mr. Right survives, Mr. Brown will survive; If both Mr Left and Mr Right survive, Mr Brown will not survive. That is, I can maintain identity and survive only if there is only one continuer and no branch; If any branch survives, that means I don’t exist. Obviously, the result of this view is that division is death, which is difficult for people to accept. Moreover, proponents of the ‘non- branching" view are more likely to regard the absence of branching as a precondition for personal identity than to really explain why there is no branching. 
The second solution to the fragmentation problem is the ‘multiple-occupancy theory’, represented by Perry. The theory is that in the case of fission, Mr. Left and Mr. Right existed before the split, and the split brain surgery merely separated them spatially. Obviously, this view leads to the paradox that how can two or even more beings coexist with a person at the same time? Thus, it is usually combined with a metaphysical claim that human beings and physical objects  are ‘four-dimensional hunks of matter’ and composed of ‘temporal parts’. This view is often referred to as ‘four-dimensionalism and is represented by Heller, Olson, and others. Strictly speaking, ‘four-dimensionalism’ should be referred to as ‘the ontology of temporal parts’, as the exploration of this theory has often caused much debate to the nature of time and change, but it does provide a better explanation of ‘multiple- occupancy theory .
This paper argues that most of the current research on the problem of personal identity is still within the scope of ‘judging human being within human being’, trying to find the answer to the problem within the scope of physical and psychological existence of human beings. However, in order to solve the problem of personal identity, a broader theoretical perspective is needed, and it is probably not enough to find the answer to the problem of personal identity only in the scope of human existence. This ‘broader theoretical perspective’ can be described in two directions. The first direction is the metaphysical direction, that is, time, that is to say, the study of personal identity and the metaphysical sense of the concept of time closely together, this direction has been representative of the theory of four-dimensionalism. The second direction can be called the physical direction, which is to combine the study of personal identity with the study of the existence of matter, and the current representative theory in this research direction is animalism. This paper criticizes the ‘non-branching’ view and the ‘multiple-occupancy theory’ to a certain extent from the point of view of the close connection between personal identity and the concept of time, pointing out the shortcomings of these two theoretical propositions, which are that they do not clarify the intrinsic connection between them and the concept of time, and also pointing out some shortcomings of four-dimensionalism. Lastly, in conjunction with the examination of the basic viewpoints of animalism, an attempt is made to put forward a viewpoint that can be called "multidimensionalism", with a view to reconciling the inconsistencies between animalism and mentalism that based on four-dimensionalism, and to provide more possibilities for the solution of the problem of personal identity.