库恩对科学进步的方式感兴趣。在他看来，科学不可能真正开始，直到大多数在一个领域工作的人都同意一个范式。在此之前，每个人都以自己的方式做自己的事情，而且你不可能拥有当今专业科学特有的协作和团队合作。一旦建立了范式理论，那么在其中工作的人就可以开始做库恩所说的“正常科学”。这涵盖了大多数科学活动。普通科学是解决特定难题，收集数据，进行计算等的业务。例如。正常科学包括：计算太阳系中每颗行星与太阳的距离，完成人类基因组图谱. 建立特定物种的进化下降。但是在科学史上经常发生的是，正常的科学抛出了异常 – 结果在主导范式内无法轻易解释。一些令人费解的发现本身并不能成为放弃已成功的范式理论的理由。但有时候莫名其妙的结果开始堆积起来，这最终导致库恩所说的“危机”。在19世纪末，无法检测到以太 – 一种看不见的媒介，用来解释光的传播方式和重力操作 – 最终导致了相对论。在18世纪，一些金属燃烧后质量增加的事实与燃素理论不一致。该理论认为可燃物质含有燃素，这是一种通过燃烧释放的物质。最终，该理论被拉瓦锡的理论所取代，即燃烧需要氧气。在范式转换期间会发生什么变化？这个问题的明显答案是，变化只是在该领域工作的科学家的理论观点。但库恩的观点比这更具激进性和争议性。他认为世界或现实不能独立于我们观察它的概念方案来描述。范式理论是我们概念方案的一部分。因此，当范式转换发生时，在某种意义上，世界会发生变化。换句话说，在不同范式下工作的科学家正在研究不同的世界。例如，如果亚里士多德看到一块石头在绳子的末端像钟摆一样摆动，他会看到石头试图达到其自然状态 – 在地面上休息。但是牛顿不会看到这个;他会看到一块石头遵守引力和能量转移的规律。或者换一个例子：在达尔文之前，任何比较人脸和猴脸的人都会被这些差异所震撼;在达尔文之后，他们会被相似之处所打动。库恩声称，在范式转变中，正在研究的现实变化是极具争议性的。他的批评者认为，这种“非现实主义”观点导致了一种相对主义，因而得出的结论是，科学进步与接近真理无关。库恩似乎接受了这一点。但他说，他仍然相信科学进步，因为他认为后来的理论通常比早期的理论更好，因为它们更精确，提供更有力的预测，提供富有成效的研究计划，并且更优雅。库恩的范式转换理论的另一个结果是科学不会以平等的方式进步，逐渐积累知识并加深其解释。相反，学科在主导范式内进行的正常科学时期与新兴危机需要新范式时的革命科学时期之间交替。这就是“范式转换”最初的含义，以及它在科学哲学中的意义。但是，当在哲学之外使用时，它通常只意味着理论或实践的重大变化。因此，引入高清晰度电视或接受同性恋婚姻等事件可能被描述为涉及范式转变。
Kuhn was interested in the way science makes progress. In his view, science can’t really get going until most of those working within a field agree upon a paradigm. Before this happens, everyone is doing their own thing in their own way, and you can’t have the sort of collaboration and teamwork that is characteristic of professional science today. Once a paradigm theory is established, then those working within it can start doing what Kuhn calls “normal science.” This covers most scientific activity. Normal science is the business of solving specific puzzles, collecting data, making calculations, and so on. E.g. Normal science includes: working out how far each planet in the solar system is from the sun, completing the map of the human genome
establishing the evolutionary descent of a particular species. But every so often in the history of science, normal science throws up anomalies–results that can’t easily be explained within the dominant paradigm. A few puzzling findings by themselves wouldn’t justify ditching a paradigm theory that has been successful. But sometimes the inexplicable results start piling up, and this eventually leads to what Kuhn describes as a “crisis.” At the end of the 19th century, the inability to detect the ether–an invisible medium posited to explain how light traveled and how gravity operated– eventually led to the theory of relativity. In the 18th century, the fact that some metals gained mass when burned was at odds with phlogiston theory. This theory held that combustible materials contained phlogiston, a substance that was released through burning. Eventually, the theory was replaced by Lavoisier’s theory that combustion requires oxygen. What changes during a paradigm shift? The obvious answer to this question is that what changes is simply the theoretical opinions of scientists working in the field. But Kuhn’s view is more radical and more controversial than that. He argues that the world, or reality, cannot be described independently of the conceptual schemes through which we observe it. Paradigm theories are part of our conceptual schemes. So when a paradigm shift occurs, in some sense the world changes. Or to put it another way, scientists working under different paradigms are studying different worlds. For example, if Aristotle watched a stone swinging like a pendulum on the end of a rope, he would see the stone trying to reach its natural state–at rest, on the ground. But Newton wouldn’t see this; he’d see a stone obeying the laws of gravity and energy transference. Or to take another example: before Darwin, anyone comparing a human face and a monkey’s face would be struck by the differences; after Darwin, they would be struck by the similarities. Kuhn’s claim that in a paradigm shift the reality that is being studied changes is highly controversial. His critics argue that this “non-realist” point of view leads to a sort of relativism, and hence to the conclusion that scientific progress has nothing to do with getting closer to the truth. Kuhn seems to accept this. But he says he still believes in scientific progress since he believes that later theories are usually better than earlier theories in that they are more precise, deliver more powerful predictions, offer fruitful research programs, and are more elegant. Another consequence of Kuhn’s theory of paradigm shifts is that science does not progress in an even way, gradually accumulating knowledge and deepening its explanations. Rather, disciplines alternate between periods of normal science conducted within a dominant paradigm, and periods of revolutionary science when an emerging crisis requires a new paradigm. So that is what “paradigm shift” originally meant, and what it still means in the philosophy of science. When used outside philosophy, though, it often just means a significant change in theory or practice. So events like the introduction of high definition TVs, or the acceptance of gay marriage, might be described as involving a paradigm shift.