Krashen's Input Hypothesis: "There is Only One Way to Acquire a Second Language"

Stephen Krashen, in the late 1970s and early 1980s, proposed a series of hypotheses that formed a highly influential theory of second language acquisition. Among them, the "Input Hypothesis" serves as the cornerstone of this framework. Its highly generalized and exclusive assertion—that comprehensible input is the only necessary condition for language acquisition—has continued to provoke in-depth discussion and empirical testing within the academic field.

斯蒂芬·克拉申（Stephen Krashen）于20世纪70年代末至80年代初提出了一系列关于第二语言习得的假说，构建了一个影响深远的理论体系。其中，“输入假说”作为该体系的核心基石，以其高度的概括性与排他性论断——即可理解性输入是语言习得的唯一必要条件——持续引发学界的深度探讨与实证检验。这一论断将语言习得的复杂过程归结为一个看似简洁的单一路径，其背后的逻辑、内涵与争议，值得我们深入剖析。

Acquisition VS Learning 

The starting point of Krashen's theory lies in the strict distinction between "acquisition" and "learning." He believes these are two completely independent mental processes.

克拉申理论的起点，在于对“习得”与“学得”的严格区分。他认为这是两种完全独立的心智过程。

"Acquisition" is a subconscious and natural process, similar to how children acquire their first language. Learners internalize the rule system of a language imperceptibly through extensive exposure in meaningful, communication-oriented contexts, ultimately developing a sense of the language. The outcome of this process is "implicit knowledge," enabling learners to use the language fluently and accurately without necessarily being able to articulate the underlying grammatical rules.

“习得” 是一个潜意识的、自然的过程，类似于儿童获得母语的方式。学习者在有意义的、以沟通为目的的语言环境中，通过大量接触语言，不知不觉地内化其规则体系，最终形成语感。这个过程的结果是“隐性知识”，学习者能够流利、准确地使用语言，却未必能说出其背后的语法规则。

"Learning," on the other hand, is a conscious and formal process that typically occurs in classroom settings. Learners systematically study language knowledge through teacher instruction, memorization of grammatical rules, and exercises. The result is "explicit knowledge," where learners can state the rules but often struggle to effectively apply this knowledge in spontaneous, rapid communication.

“学得” 则是一个有意识的、正式的过程，通常发生在课堂环境中。学习者通过教师讲解、记忆语法规则、做练习等方式，系统地学习语言知识。其结果是“显性知识”，学习者能够陈述规则，但在自发的、快速的交流中，往往难以有效调用这些知识。

Why is input key to acquisition?

Krashen opposed the then-prevalent "Output Hypothesis" (which posited that speaking and writing practice directly promotes language development), arguing that output is merely the result of acquisition, not its cause. He maintained that improvements in language ability come not through practicing production, but through comprehending input. When a learner's brain focuses on the meaning of the message rather than the form of the language, the internal language acquisition mechanism is activated. This process of absorbing linguistic structures by understanding meaning is the only way acquisition occurs.

克拉申反对当时盛行的“输出假说”（即认为多说多写能直接促进语言能力发展），认为输出只是习得的结果，而非原因。语言能力的提升，并非通过练习产出，而是通过理解输入的信息来实现。当学习者的大脑专注于信息本身，而非语言形式时，内在的语言习得机制才会被激活。这种通过理解意义来吸收语言结构的过程，是习得发生的唯一方式。 翻译为英文

What is Comprehensible Input?

The essence of the "Input Hypothesis" lies in its definition of input quality, known as "comprehensible input." Krashen illustrated this concept using the famous "i+1" formula.

“输入假说”的精髓在于对输入质量的界定，即“可理解性输入”。克拉申用著名的 “i+1”公式 来阐述这一概念。

• "i" represents the learner's current language proficiency level.
• "1" refers to language knowledge slightly beyond the learner's current ability—specifically, a small amount of new, unacquired linguistic structures.
• "i+1," therefore, denotes input that learners can largely understand (with the aid of context, situational cues, non-verbal information, etc.), yet contains just enough challenging material to stretch their existing capabilities.

“i” 代表学习者当前的语言水平。
“1” 代表略高于学习者当前水平的语言知识，即包含少量新的、尚未掌握的语言结构。
“i+1” 则是指那些学习者能够基本理解（借助上下文、情境、非语言信息等），但其中又包含足以挑战其现有水平的语言材料的输入。

Controversy

Although Krashen's Input Hypothesis is highly insightful and has profoundly influenced practices such as communicative language teaching, its rigid assertion of being the "only" way to acquire language has made it a focal point of academic debate.

尽管克拉申的输入假说极具启发性，并深刻影响了交际教学法等实践，但其“唯一性”的强硬论断也使其成为学界争议的焦点。

Neglect of the Role of Output: Many researchers argue that output is not merely the result of acquisition but can also drive it.

对输出作用的忽视：许多研究者提出，输出不仅是习得的结果，也能推动习得。

Example: When a learner wants to say, "I went to the bookstore yesterday," but does not know how to express "yesterday" and "go" together, they might produce a sentence like, "I yesterday go bookstore." This failure in output may prompt them to pay closer attention to how native speakers express the idea or to actively seek the correct past tense form, "went," in subsequent input. This type of "pushed" learning is difficult to achieve through input alone.

例子：当一个学习者想说“我昨天去书店”却不知道“昨天”和“去”的搭配时，他可能会说出“我昨天go书店”这样的句子。这种尝试输出的失败，会促使他更有意识地注意母语者如何表达，或在后续输入中寻找“went”这个正确的过去式形式。这种“被推动”的学习是纯粹输入难以实现的。

Practical Challenges of "i+1"​: The "i+1" concept is descriptive but difficult to precisely define and implement in practice.

“i+1”的可操作性：“i+1”是一个描述性概念，在实践层面难以精确界定和提供。

Example: In a classroom of 30 students, each learner’s "i" is different. It is nearly impossible for a teacher to provide precisely tailored "i+1" input for every individual. As a result, teaching materials and instruction may end up being "i+0" (too easy) for some students and "i+5" (too difficult) for others.

例子：在一个有30名学生的班级里，每个学生的“i”都不同，教师几乎不可能为每个人提供精准的“i+1”输入。最终，教材和教学很可能变成对部分学生来说是“i+0”（太简单），对另一部分学生来说是“i+5”（太难）的尴尬局面。

Complete Dismissal of Explicit Knowledge: A substantial body of empirical research suggests that, in adult second language acquisition, explicit knowledge can become automated through practice and may transform into implicit knowledge.

对显性知识的完全否定：大量实证研究表明，在成人二语学习中，显性知识通过练习可以自动化，并能转化为隐性知识。

Example: A learner consciously memorizes the rule of the present perfect tense ("have/has + past participle") in a classroom setting. Through extensive reading and writing practice, they eventually become able to use "I have finished my work" naturally in speech without consciously thinking about the rule. This demonstrates that "learned" knowledge can, under certain conditions, be converted into "acquired" ability.

例子：一个学习者通过课堂学习，有意识地记住了“现在完成时”的规则（have/has + 过去分词）。通过大量的阅读和写作练习，他最终能够在不思考规则的情况下，在口语中自然地使用“I have finished my work”。这表明“学得”的知识在一定条件下可以转化为“习得”的能力。 翻译为英文