There are times when lawyers and judges have to grapple with the adequacy of grounds for the purported exercise of official powers such as arrest, search, or interception. These activities usually require "reasonable grounds" to "believe" that an offence has been committed or that evidence will be found, or that a person has committed an offence.
Some jurisdictions describe adequate grounds differently, but again the problem is to distinguish between adequate and inadequate grounds. "Probable cause" is the description of the requirement of adequate grounds for search in the USA, although in the context of searches of pupils at schools (not private schools) a lower standard, called "reasonable suspicion" is used to describe adequate grounds.
The distinction between reasonable grounds to believe (which constitutes adequate grounds) and reasonable grounds to suspect (which is usually inadequate), performs the same function as the distinction between "probable cause" (adequate grounds) and "reasonable suspicion" (usually inadequate, but adequate for government school officials to search pupils in the USA).
Whichever terms are used, it is tempting to say that the concepts are fluid and take their content from their context: Ornelas v United States, 517 U.S. 690, 696 (1996). This view was endorsed yesterday in Safford Unified School District #1 v Redding  USSC No 08-479 (25 June 2009).
Lest the subject become impossibly vague – as it would if one were to accept that all the concepts are fluid - it may be useful to suggest a metaphor (admittedly a bit silly too, for serious-minded people): adequate grounds are like a vessel filled to a sufficient level: the shape of the vessel is fixed, the required level is marked on it, and it is the ability of the fluid to reach that mark that is critical.
For example, "probable cause" is described this way:
"Probable cause exists where 'the facts and circumstances within [an officer's] knowledge and of which [he] 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," Brinegar v. United States, 338 U. S. 160, 175–176 (1949) (quoting Carroll v. United States, 267 U. S. 132, 162 (1925)), and that evidence bearing on that offense will be found in the place to be searched." [Safford, Souter J, slip op pp 3-4]
The first part of this describes the fluid ("the facts ... information"), and the second the vessel ("warrant ... committed"). The man of reasonable caution is, of course, the judge (pretending to imagine what some other reasonable person would decide – the objective person – but really deciding what he thinks is reasonable). Reasonableness comes into it twice: in relation to the fluid ("reasonably trustworthy information") and in relation to the vessel ("sufficient to warrant a man of reasonable caution ...").
More on the vessel: the enforcement officer must have information that provides "a fair probability" or a "substantial chance" that evidence will be discovered: Safford, Souter J pp 4-5, citing Illinois v Gates, 462 U.S. 213, 230 (1983). These terms reflect, in this metaphor, the level to which the vessel must be filled.
I think the metaphor helps by preventing the confusion that would arise if "a fair probability" and "a substantial chance" were to be applied to the quality of the information, which must be "reasonably trustworthy". It is only when satisfied of the reasonable trustworthiness of the information that one proceeds to ask what it suggests. Check the quality of the fluid, then what it does in the vessel. This approach has general application, translatable to the terminology of other legal systems.
Safford Unified School District #1 v Redding is a civil case, dealing with privacy rights, and would not normally come under consideration here. I have previously mentioned searches of school pupils in Canada: see notes for 28 April 2008, and Safford is interesting for its increase in quality of information needed to authorise a strip search of a pupil, compared with the lesser quality sufficient for search of bag or pockets. Thomas J, dissenting on the reasonableness of the strip search here, would have upheld it on the basis that departure from "bedrock" Fourth Amendment law was not appropriate.
Anyone who has been following developments in standard of proof jurisprudence (see “Balance of probabilities” in Index) will have noted differences of approach: the standard can shift or the quality of the evidence can change, according to what has to be decided. This is analogous to the position just mentioned: for strip searches of pupils, on the majority approach in Safford, does the standard change or does the quality of the information change? Souter J, for the Court, regarded the standard as fixed, requiring for strip searches a higher quality of information to meet it. Some courts would say that if information is sufficient to reach a given standard of persuasion, it is always sufficient to reach that standard, regardless of the seriousness of the issue; if context requires caution it is the standard that should be increased.