But the memo, signed by Assistant Attorney General Virginia Seitz, relies heavily on past opinions to argue that a "recess" is not determined by the technicality of whether a recess has been agreed to — instead, the memo argues it has more to do with whether the Senate is available to fulfill its constitutional role of providing advice on appointees.

For example, the memo notes that in 1921, Attorney General Harry Daughterty made this distinction plain when he wrote: "Regardless of whether the Senate has adjourned or recessed, the real question … is whether in a practical sense the Senate is in session so that its advice and consent can be obtained."

The memo also notes that 100 years ago, the Senate Judiciary Committee based the definition of "recess" on the Senate's ability to conduct business.

"Guided by these principles, we conclude that the President may determine that pro forma sessions at which no business is to be conducted do not interrupt a Senate recess for purposes of the Recess Appointments Clause," Seitz wrote.

To bolster that opinion, the memo noted that the Senate has not been around to conduct business during the pro forma sessions, many of which last just a few seconds. It also said giving the Senate the ability to block recess appointments simply by holding non-working sessions to prevent the president from making any recess appointments at all would "raise constitutional separation of powers concerns."

The memo does acknowledge that the Senate could hold continuous working sessions in order to block recess appointments, but said the president "may properly determine that the Senate is not available under the Recess Appointments Clause when, while in recess, it holds pro forma sessions where no business can be conducted."

Justice considered several counterarguments to its conclusion, such as the fact that the Senate in December approved legislation during some of the pro forma sessions. However, it rejected the idea that those events show the Senate is actually available to provide advice and consent.

"We do not believe, however, that these examples prevent the President from determining that the Senate remains unavailable to provide advice and consent during the present intrasession recess," it said. "The scheduling order under which the pro forma sessions are held during this recess expressly provides that there is to be 'no business conducted.' "

In addition to its conclusion that the Senate is in recess, Justice also cited several historical cases demonstrating the widely held view that recess appointments during an intrasession break are constitutional. Justice said the combination of these two findings justify Obama's recess appointments last week.

"In our judgment, the text of the Constitution and precedent and practice thereunder support the conclusion that the convening of periodic pro forma sessions in which no business is to be conducted does not have the legal effect of interrupting an intrasession recess otherwise long enough to qualify as a 'Recess of the Senate' under the Recess Appointments Clause," it concluded.

However, the memo did warn that there is "limited judicial authority" on whether these recess appointments could be successfully challenged in court.

"Due to this limited judicial authority, we cannot predict with certainty how courts will react to challenges of appointments made during intrasession recesses, particularly short ones," it said.

Republicans have warned that the appointments will likely be challenged, although many believe the basis of any challenge will have to come from a private company that argues it was harmed through the actions of the appointees.

Obama last week appointed Richard Cordray to head the Consumer Financial Protection Bureau, as well as three nominees to the National Labor Relations Board.