Definition
In criminal law, particularly the law of homicide, cool blood refers to a mental state characterized by calmness, tranquility, and the undisturbed exercise of one's rational faculties at the time of a killing. A person acts in cool blood when they are free from violent passion, fury, or uncontrollable emotional excitement—capable of deliberate thought and purposeful action.
Cool blood is not a standalone offense or defense. It is a condition of mind used to determine the degree of culpability in a homicide. A killing done in cool blood is evidence of premeditation and deliberation, pointing toward first-degree or aggravated murder. A killing done in the absence of cool blood—that is, in the heat of passion—may reduce the offense to a lesser degree of homicide or manslaughter.
Common Language
Modern common usage (Wiktionary): "Cool blood" or "cold blood" appears in common English as a shorthand for emotional detachment or lack of feeling—often in the phrase "in cold blood," meaning deliberately and without remorse.
Historical common usage (Webster's 1913): "Cool" signifies freedom from excitement or passion; composed; deliberate. "Blood" in older usage carried strong connotations of temperament or passion.
The legal meaning tracks the ordinary meaning more closely than many legal terms do, but the gap is important: common usage treats "cold blood" as a moral descriptor (callousness, remorselessness), while legal usage treats "cool blood" as a cognitive and volitional descriptor—it is about the presence of rational control, not the absence of human feeling. A defendant could kill with evident regret and still be found to have acted in cool blood if reason was undisturbed.
Common Confusion
Cool blood is frequently paired with—and implicitly defined against—its counterpart, heat of passion (or hot blood). The two are not simply opposites on a spectrum; they are legal conditions with different doctrinal effects. Heat of passion is an affirmative mitigation doctrine with recognized elements. Cool blood, by contrast, is the baseline presumptive state against which passion is measured. Researchers should not treat cool blood as an element the prosecution must independently prove; it is typically the default condition absent evidence of provocation.
Cool blood should also be distinguished from cold blood, a phrase that appears in popular usage and occasionally in older judicial opinions. The two phrases overlap but are not interchangeable in legal analysis. "Cold blood" emphasizes premeditation and cruelty; "cool blood" emphasizes the rational, unpassioned state of mind at the moment of the act.
Why It Matters in Research
Researchers working in historical homicide law will encounter cool blood as a central analytical term in cases and treatises from the eighteenth and nineteenth centuries, when common law courts relied heavily on the cool blood / heat of passion distinction to sort killings into murder and manslaughter. The term appears less frequently in modern statutory frameworks, which typically codify the distinction through express elements of deliberation, premeditation, and provocation rather than using the phrase itself.
Three traps for historical researchers:
First, the phrase's absence in modern sources does not mean the concept disappeared. Modern statutes and pattern jury instructions encode the same idea under different vocabulary—look for "deliberation," "premeditation," and the negation of "adequate provocation."
Second, the threshold for what qualifies as passion sufficient to displace cool blood shifted considerably across jurisdictions and time periods. Nineteenth-century courts varied widely on whether words alone, as opposed to physical provocation, could negate cool blood. Opinions using identical language may reflect materially different standards.
Third, cool blood was sometimes treated as a temporal question (how much time passed between provocation and the act?) and sometimes as a psychological question (what was the actual state of mind?). These two framings produce different analytical frameworks. Historical sources conflate them frequently; researchers should track which framing a given court is applying.
Historical Dictionary Support
The three source dictionaries are in close agreement. Black's (both editions) and Bouvier's offer functionally identical definitions centered on calmness, tranquility, and the undisturbed use of reason. Bouvier adds the practical framing most useful to researchers: ascertaining whether a killing was done in cool blood is necessary to determine the degree of guilt—making clear that cool blood is an analytical tool for grading offenses, not a freestanding legal element.
Bouvier cites Bacon's Abridgment (Murder, B) and older English reporters (Kelynge, Siderfin, Levinz), which situates the term firmly in the common law inheritance. Researchers using these sources should be aware that the English common law framework underlying these citations preceded the American statutory codification of murder degrees, which began in the late eighteenth century. The concept of cool blood in those early sources operates in a doctrinal context somewhat different from the American first-degree / second-degree murder structure that later adopted and adapted the terminology.
None of the historical dictionaries address what happens at the margin—partial cooling, interrupted passion, or renewed passion after a period of calm. These edge cases generated significant case law and are not resolved by the dictionary definitions.
Jurisdictional Note
American jurisdictions vary in how explicitly they preserve the cool blood language versus translating it into statutory elements. States following the Model Penal Code's culpability framework largely replaced the cool blood / heat of passion binary with the extreme emotional disturbance standard, which is broader and does not map directly onto the old categories. Researchers should not assume that cool blood doctrine and EED doctrine are equivalent.