Likewise, when the Prophet sallallaahu alayhi wa sallam does something as an act of ibaadah (worship), but he does not command its performance, then what is correct is that this action of his is indicative of it being mustahabb (recommended). If he does something as an act of aadah (custom or habit), then it is indicative of it being mubaah (permissible).
Whatever the Prophet sallallaahu alayhi wa sallam acknowledges of statements and actions, then the ruling is one of ibaahah
(permissibility), or other than it, according to the manner in which he acknowledged such statements and actions.
As for the ijmaa (consensus): it is the agreement of the mujtahid Scholars upon a new judgement. So, whenever we are certain about their ijmaa, then it is obligatory to turn to it, and it is not lawful to oppose. It is necessary that any ijmaa be rooted in the evidences of the Book and the Sunnah.
As for qiyaasus-saheeh (correct and sound analogy): it is linking a subsiduary branch with its root, due to a commen Illah (effective cause) between them. So whenever the Lawgiver indicates a matter, or describes it with a particular wasf (characteristic); or the Scholars deduce that the ruling has been legislated because of that particular wasf (characteristic), then if that particular wasf
(characteristic) is found to exist in another issue, which the Lawgiver has not legislted any particular ruling for - without their being a difference between it and the texts - then it is obligatory to link the two in their ruling. This is beacuse the All-Wise Lawgiver does not differentiate between matters equivilent in their characteristics, just as He does not join betweeen dissimilar and opposing matters.
This sound and correct qiyaas (analogy) is al-Meezaan (the Balance) which Allaah sent down. And it is inclusive of justice, and it is that by which justice is recognised.
Qiyaas is only resorted to when there exists no text. So this asl (fundamental principle) is turned to when there exists no other source.. And qiyaas supports the text. Thus, all that the texts that the Lawgiver has given rulings to, then they are in agreement with qiyaas, not in opposition to it.
The Scholars of usool have deduced from the Book and the Sunnah many usool (fundamental principles), upon which they have built many rulings; by which they have also benefitted themselves and benefitted others.
Amongst these usool (fundamental principles):
 Al-yaqeen laa yazoolu bish-shakk [certainty is not invalidated by doubt].
Under this principle they have entered many ibaadaat (acts of worship), muaamalaat (social interactions) and huqooq (rights). So whosoever entertains a doubt regarding any of that, should return to the asl of
certainty. They also deduce: al-aslut-tahaarah fee kulli shay [The basis concerning all things is that they are pure]. And: al-aslul-ibaahah illaa ma dallad-daleelu alaa najaasatihi aw tahreemihi [The asl (basic principle) is permissibility of using anything, except when there exists an evidence indicative of its impurity or prohibition]. And: The asl is freedom from accountability concerning obligations towards the creation, until a proof is established to the contrary. And: The asl is the continuation of accountability concerning the obligations to the Creator, and to His servants, until there is certain proof of freedom and discharging.
 And from them is that: Al-mashaqah tajlibut-tayseer [difficulty brings about ease].
Based upon this are all the rukhas (concessions) allowed during a journey, and a lightening of Ibaadaat (acts of worship), muaamalaat (social transactions), and other matters.
 And from it is their saying: Laa waajib maal-ajaz wa laa muharram maad-daroorah [there is no obligation with inability, nor is there any prohibition with necessity].
The Lawgiver has not made incumbant upon us what we are incapable of doing in totality. What the Lawgiver has made obligatory, from the obligations, but the servant is incapable of performing it, then that obligation is totly dropped. However, if he is capable of performing a part of it, then it is required for him to fulfill what he is capable of, whilst the part he is incapable of is dropped. There are many mnay examples of this.