Writing for the Court, Justice Charles Evans Whittaker found that a Officer Marsh had probable cause and reasonable grounds to arrest Draper without a warrant based on Hereford’s tip, which was subsequently verified in every detail except for the actual possession of narcotics. Whittaker first addressed the Draper’s argument that Hereford’s tip was hearsay that couldn’t be legally considered in determining probable cause. Relying on Brinegar v. United States (1949), Whittaker explained that there is a major distinction between the admissibility of evidence to prove guilt at trial and the facts relied upon to show probable cause, writing that there is “a like difference in the quanta and modes of proof required to establish them.” He emphasized that requiring technically competent evidence to show probable cause would be a “wholly unwarranted emphasis” that would practically require proof sufficient to establish guilt in order to justify an arrest or search.

Regarding whether Hereford’s tip provided Marsh with sufficient information, Whittaker emphasized that Hereford had been employed by law enforcement, who had always found his information to be accurate and reliable. Whittaker asserted that Marsh “would have been derelict in his duties had he not pursued it.” By observing a man matching the exact physical attributes, wearing the precise clothing, carrying a tan zipper bag, and walking at a fast pace from the location Hereford said Draper was traveling from, Whittaker argued that Marsh “personally verified every facet of the information given him by Hereford except whether petitioner had accomplished his mission and had the three ounces of heroin on his person or in his bag.” With every other detail verified, Whittaker found Marsh possessed reasonable grounds to believe that Draper would have the heroin with him.

To define the constitutional standard, Whittaker explained that “[i]n dealing with probable cause, . . . as the very name implies, we deal with probabilities. These are not technical; they are the factual and practical considerations of everyday life on which reasonable and prudent men, not legal technicians, act.” Whittaker emphasized that probable cause is established when “the facts and circumstances within [the arresting officers’] knowledge and of which they had reasonably trustworthy information [are] sufficient in themselves to warrant a man of reasonable caution in the belief that’ an offense has been or is being committed.” Ultimately, Whittaker concluded that Hereford’s tip met this standard, meaning that the warrantless arrest of Draper was lawful and the subsequent search was validly made incident to that lawful arrest.