A System of Logic: Ratiocinative and Inductive Informative Summary


Volume II of John Stuart Mill’s seminal work, “A System of Logic: Ratiocinative and Inductive,” continues his exploration of inductive reasoning. It builds upon the foundation laid in Volume I, dissecting the nuances of scientific methodology, the role of hypotheses, and the nature of empirical laws. Mill distinguishes between ultimate and derivative laws of nature, arguing that while scientific progress aims to resolve complex laws into simpler ones, there are inherent limits to this reductionist approach. He emphasizes the critical role of hypotheses in scientific discovery, advocating for their provisional use as tools for guiding investigation and generating testable predictions.

The text also delves into the concept of chance and its elimination in scientific inquiry, examining how the frequency of coincidences can be used to infer the existence of underlying laws. Mill provides a rigorous analysis of the calculation of chances, highlighting its limitations and the importance of relying on solid causal reasoning rather than mere numerical probabilities. He further explores the extension of derivative laws to adjacent cases, examining the role of analogy in scientific reasoning and its value in suggesting new lines of investigation.

Key Findings:

  • Distinction between ultimate and derivative laws of nature.
  • Importance of hypotheses in scientific discovery, but the need for their verification.
  • The role of chance in scientific inquiry and how to eliminate its effects.
  • The limitations of the calculation of chances and the importance of causal reasoning.
  • The extension of derivative laws to adjacent cases and the role of analogy in scientific reasoning.


  • Ultimate vs. Derivative Laws: The reader will learn the crucial difference between ultimate laws, which are fundamental and irreducible, and derivative laws, which can be resolved into simpler, more general laws. This understanding is fundamental for comprehending the hierarchical structure of scientific knowledge.
  • Hypothetical Method: The text elucidates the Hypothetical Method in science, where provisional assumptions are made to guide research. The reader will learn the criteria for a legitimate hypothesis, emphasizing the importance of testability and the possibility of verification or refutation.
  • Elimination of Chance: Mill explains the concept of chance as the absence of a known causal connection and presents methods for eliminating its influence in scientific experiments. The reader will learn how to discern between casual coincidences and genuine causal relationships by analyzing the frequency of events.
  • Calculation of Chances: The reader will delve into the principles of probability, learning how to calculate chances and evaluate the likelihood of events. Mill emphasizes the limitations of relying solely on numerical probabilities, stressing the importance of understanding the underlying causal mechanisms.
  • Analogy in Science: The text explores the role of analogy in scientific reasoning, showing its usefulness in suggesting new hypotheses and lines of investigation. The reader will learn how to assess the strength of an analogical argument, considering both the similarities and differences between the compared cases.

Historical Context:

This volume was published in 1868, amidst a period of significant scientific advancement and intellectual ferment. Darwin’s theory of evolution, published a decade earlier, had revolutionized biological thinking, and the scientific community was grappling with its implications. Mill’s work reflects this intellectual climate, emphasizing the importance of empirical investigation, rigorous methodology, and the constant refinement of scientific knowledge. His emphasis on the limitations of purely hypothetical reasoning and the importance of verification reflects a growing awareness of the need for robust evidence-based science.


  1. Ultimate Laws: Ultimate laws of nature are fundamental principles that cannot be further reduced to simpler laws. Example: the law of universal gravitation.
  2. Derivative Laws: Derivative laws are deducible from and can be resolved into more general, ultimate laws. Example: Kepler’s laws of planetary motion.
  3. Hypotheses: Hypotheses are provisional assumptions made to explain phenomena and guide scientific inquiry. Example: the hypothesis of a luminiferous ether to explain the propagation of light.
  4. Verification: A key characteristic of a scientific hypothesis is its testability and the possibility of its verification or refutation through observation and experiment.
  5. Chance: In the context of scientific inquiry, chance refers to the absence of a known causal connection between events.
  6. Elimination of Chance: Scientists employ methods like repeated trials and statistical analysis to eliminate the influence of chance and isolate the effects of constant causes.
  7. Empirical Laws: Empirical laws are observed uniformities for which the underlying causal mechanisms are not yet understood.
  8. Limits of Empirical Laws: Empirical laws are reliable only within the observed limits of time, place, and circumstance and should not be extended beyond those boundaries without further investigation.
  9. Approximate Generalizations: Approximate generalizations are statements true of most but not all members of a class. Example: Most birds can fly.
  10. Probable Evidence: Probable evidence is based on approximate generalizations and provides a degree of support for a conclusion, but not absolute certainty.
  11. Coincidences: Coincidences are unexpected regularities in seemingly random events.
  12. Credibility of Coincidences: The credibility of an alleged coincidence depends on factors like the reliability of the witness, the possibility of other explanations, and the degree to which it deviates from expected randomness.
  13. Resemblance: Resemblance between phenomena can be ascertained directly through observation or indirectly through inference and the application of general laws.
  14. Mathematical Laws: Mathematical laws are universal truths about equality, inequality, and proportionality that apply to all phenomena regardless of their origin.
  15. Deductive Nature of Mathematics: Mathematics is largely deductive, with its vast body of theorems derived from a small number of axioms and definitions.
  16. Geometry as Physical Science: Geometry is a physical science, with its theorems representing laws of external nature, even though they can be deduced from abstract premises.
  17. Disbelief: Disbelief is a firm conviction that an assertion is false, even in the face of seemingly strong evidence.
  18. Grounds for Disbelief: The grounds for disbelief may include the improbability of an event, its contradiction of established laws of nature, or the unreliability of the evidence supporting it.
  19. Improbability vs. Impossibility: An event is improbable if it has a low likelihood of occurrence, while it is impossible if it contradicts a well-established law of nature.
  20. Miracles and Causation: Miracles are not necessarily contradictions to the law of causation but rather represent new effects produced by the intervention of a supernatural cause.


  1. Induction: The process of reasoning from specific observations to general principles.
  2. Deduction: The process of reasoning from general principles to specific conclusions.
  3. Causation: The relationship between cause and effect, where the cause is a necessary and sufficient condition for the effect.
  4. Hypothesis: A provisional explanation for a phenomenon that can be tested through further investigation.
  5. Empirical Law: A regularly observed pattern in nature for which the underlying cause is not yet understood.
  6. Chance: The absence of a known causal connection between events.
  7. Probability: The likelihood of an event occurring, expressed as a numerical value between 0 and 1.
  8. Analogy: A comparison between two things that are similar in some respects, used to infer further similarities.
  9. Coincidence: An unexpected concurrence of events, often appearing to have a pattern or significance.
  10. Disbelief: The rejection of an assertion as false, based on evidence or reasoning.


  1. Falling Bodies: Mill uses the example of falling bodies to illustrate the concept of a derivative law, showing how the observed uniformity of their acceleration can be deduced from the law of universal gravitation.
  2. Planetary Motion: Kepler’s laws of planetary motion are cited as a classic example of derivative laws that can be resolved into the more fundamental law of gravitation.
  3. Luminiferous Ether: The hypothesis of a luminiferous ether to explain the propagation of light serves as an example of a scientific hypothesis that was initially useful but later superseded by more comprehensive theories.
  4. Loaded Dice: Mill uses loaded dice to illustrate the elimination of chance, showing how repeated trials can reveal the influence of a constant cause (the loading) amidst the variability of individual throws.
  5. Black Crows: The assertion “All crows are black” is used to illustrate an empirical law based on simple enumeration. The possibility of a white crow highlights the limitations of such generalizations.
  6. Tides: The simultaneous occurrence of high tides at opposite points on Earth exemplifies a uniformity of coexistence that can be explained by the laws of gravitation.
  7. Binomial Theorem: The binomial theorem in algebra demonstrates the identity of different modes of forming a number, showcasing the deductive nature of mathematical reasoning.
  8. Trigonometrical Survey: Mill explains how trigonometrical surveys utilize geometric principles to indirectly measure distances and angles, demonstrating the practical application of mathematical laws.
  9. Miracles: The debate about the credibility of miracles is used to illustrate the distinction between improbability and impossibility, highlighting the importance of considering counteracting causes and the prior belief in supernatural agency.
  10. King of Siam and Ice: The anecdote of the Dutch travelers trying to convince the King of Siam about the existence of ice exemplifies the challenges of accepting assertions that contradict one’s prior experience, even when those assertions are true.


John Stuart Mill’s analysis of induction, chance, and the grounds for disbelief offers a profound exploration of the principles underlying scientific reasoning and the evaluation of evidence. He argues for a rigorous approach to scientific inquiry, emphasizing the importance of verification, causal reasoning, and the constant refinement of knowledge through ongoing observation and experimentation. While acknowledging the role of hypotheses and approximate generalizations in navigating practical situations, Mill ultimately champions a commitment to seeking universal truths and grounding our beliefs in the most comprehensive and reliable evidence available. His work remains a vital contribution to the philosophy of science, offering insights relevant not only to scientists but to anyone seeking to understand the world through reason and experience.

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