Evidence leaves the matter in doubt
The burden of proof (onus probandi) can be defined as a legal duty upon a party to prove or disprove a disputed fact or facts during the course of litigation  . This means in general terms, that if on any issue of fact if no evidence is led, or the evidence leaves the matter in doubt, the party upon whom the burden of proof rests has not discharged it, and accordingly fails on that issue  . It makes no difference to his failure that his opponent may not have proved his own averments  .
Scottish criminal courts place the burden of proof on the prosecution at all times  . This is because in criminal proceedings the court must assume innocence  . The accused cannot be found guilty by a court unless the prosecution provides evidence to prove his guilt; and so in course, he cannot be punished by a criminal court unless he has first been found guilty  . This presumption of innocence is unqualified applying to everyone who is charged with a criminal offence regardless of their character  . This point of law has been articulated in Slater v HM Advocate.
“The jury was told that what is familiarly known as the presumption of innocence in the criminal cases applied to the appellant (in light of his ambiguous character) with less effect than it would have applied to a man whose character was not open to suspicion. This amounted, in our opinion, to a clear misdirection in law. The presumption of innocence applies to every person charged with a criminal offence in precisely the same way, and it can be overcome only by evidence relevant to prove crime with the commission of which charged." 
It is because of this active presumption of innocence in criminal trials, the burden of proof lies with the Crown to prove the accused’s guilt beyond a reasonable doubt  . If evidence is lead by the prosecution a reasonably doubt exists as to the accused’s guilt, he must be acquitted  . The evidential burden of proof stays with the Crown throughout the whole of the trail. Even in the scenario where the accused is pleading a line of defence under common law or statute the burden remains the same  . It is said to be an “a grave misdirection to suggest that the accused carries a burden of proving innocence." 
There is has been a tendency, particularly on the part of English legal writers, to distinguish what they call the “legal" burden proof from what they call the “provisional" burden  . The legal burden is the requirement on the prosecution to present evidence before the court to entitle it to convict  . There comes however at which the evidence led by the prosecution is sufficient for the accused to be convicted unless he leads convincing evidence is led in his defence. This is the “provisional" burden. A case where such a burden did arise is Milne v Whaley in which the accused was acquitted by a sheriff on a charge of driving without a licence or insurance because there was no corroboration  . The High Court held that the on appeal the Sheriff’s approach had been incorrect, saying:
“All the Crown has to do is to demonstrate prima facie the absence of an entitlement to drive and the Crown has amply done that in this case by proving the circumstances in which the charge was brought. There so if an accused person wishes to displace the prima facie inference... it is for him to do so." 
Another example where a provisional burden may arise is in circumstances, where the accused is found in “recent possession" of stolen goods may to prove that he is not guilty of theft or reset  .
“If the rule is to have full effect in shifting the onus from the prosecution to the accused and raising a presumption of guilt which the accused must redargue or fail, three conditions must concur:- (a) that the stolen goods should be found in the possession of the accused; (b) that the interval between the theft of the goods and their discovery in the accused’s possession should be short… and (c) that there should be “other criminative circumstances “over and above the bare fact of actual possession"  .
This may be an unfortunate shift on the onus of proof, although it has been repeated in the courts since, in Cryans v Nixon  . A garage owner was charged with the theft of motor accessories and scrap metal. The accused's premises were about 200 yards from the premises from which the goods had been stolen; tracks could be seen leading from those premises to the accused's premises. On a search being made, some of the stolen property was found in one of the accused's sheds and that the accused, when asked for an explanation, replied that he knew nothing about the property in question  . The Sheriff applied application of the doctrine of recent possession, rejected the explanation, and convicted the accused. On appeal it was that held that the proved facts did not entitle the Sheriff to view the case as one in which there was an onus on the accused to rebut the presumption of guilt arising from the possession of recently stolen goods and conviction quashed.
It is in McDonald v HM Advocate Lord Justice General Hope where it is articulated that the doctrine of recent possession “cannot be confined within the ordinary rule that the onus of proof remains throughout on the Crown.  " In principle, the legal burden rests with the prosecution but can be discharged by proof of “recent possession" in this way, and in such circumstances all that shifts is the “provisional burden"  .
The courts also recognise that where an explanation for otherwise suspicious conduct lies peculiarly within the knowledge of the accused a provisional burden may fall upon him to explain his conduct  . In HM Advocate v Hardy, the accused was charged with fraud by pretending to be the husband of a deceased woman and as such entitled to legal rights in her estate  . The jury, took note of the fact that the accused had not gone into the witness-box to say where he had met the deceased, where they were married or what had happed to the men he claimed had acted as witness at the ceremony  . Lord Justice-Clerk Aitchison articulated;
“There are certain cases in which the proved facts may raise a presumption of guilt, and in which, in the absence of some explanation by the person accused, - where the person accused is the one person who can know the real truth – a jury man may be entitled to proceed to draw an inference of guilt; and I direct you in law that this is one of them" 
At first sight, such a principle may be thought to be at odds with the right to silence  . An answer to this “conflict" is suggested by Mochan v Herron, where Sheriff Peterson observed that an accused in suspicious circumstances was fully entitled to remain silent (unlike other witnesses, who can be held in contempt of court for not testifying) but if the accused did remain silent there could be no objection to the court drawing its own conclusion from the evidence  .
The defence of insanity and diminished responsibility place a temporary persuasive burden on the accused. When submitting a special defence there is duty for the accused party to provide advance notice of such an intention  . At common law, alibi, incrimination, and self defence are considered as special defences  . By statute, automatism, coercion and consent in sexual offences are considered special defences. 
As a special defence if insanity is proven, the accused will be acquitted  . Normally expert evidence is given to support such an assertion  . Unlike insanity, diminished responsibility is not a special defence. If successfully pleaded by the accused diminished responsibility will not lead to an acquittal rather it will lead to a reduction of the charge of murder to culpable homicide  . At common law there is a presumption of sanity that must be rebutted by the party alleging insanity or diminished responsibility  . A persuasive burden is placed on the accused from the outset to prove insanity or diminished responsibly is invoked as a defence  . The standard of proof of the provided evidence in support of these defences is on balance of probabilities at most  . These defences would be difficult disprove, particularly so as the prosecution do not have the mandate to impose that the accused submit to psychiatric examination  . Perhaps it is for reasons of convenience that that burden is placed with the accused  .
The Criminal Justice and Licensing (Scotland) Act 2010 which received Royal Assent on the 6th of August this year, accounts for these expectations through statute  . However the term insanity is not present in the Bill, rather headed as “criminal responsibility of persons with mental disorder". The Act does not amend the burden of proof for these defences and the general common law rules will still stand  .
Civil cases often work by the prescribed principle that the burden of proof “rests with the party who would fail if no evidence were adduced on either side  ". Hence the burden of proof normally lies with the pursuer  .
At different points of a civil trial, the burden of proof often switches between parties in respect of different issues  . In cases of negligence, the pursuer will bear the burden of presenting evidence that the defender has been negligent. However, should the defender want to plea a defence such as contributory negligence, or Volenti non fit Injuria (to a willing person, no injury is done), then he will word missing the burden of proof in that issue. This is because a party to a civil case will not normally be required to prove a negative – and so it is for the defender to prove these defences, not for the pursuer to disprove them  .
Statutory provisions can place a burden of proof but alter who the burden of proof lies with. Nimmo v Alexander Cowan & Sons Ltd is the leading case highlighting this issue  . In Nimmo there was an alleged breached of statutory duty under the Factories Act 1967. S 29 (1) of the Act sets out that a factory “shall, far as reasonably practicable, be made and kept safe for any person working there"  . It was held it was not for the purser to submit evidence that premises were not safe, but not that it was not “reasonably practicable" to make them safe  . If the defender wished reply upon this proviso, he bore the burden of proof in that respect. In the words of Lord Wilberforce, “exceptions, ect. are to be set up by those who rely on them  .
On regarding the notion who asserts must prove my research supports this as a general principle. In criminal cases the legal burden of proof remains with the prosecution to prove the accused’s guilt beyond a reasonable doubt and is based on sound ethical principles of society. In some circumstances whether through common law or statue a provisional burden may be placed upon the accused to account for his actions. In civil law the pursuer is normally charged with the burden of proof, but the defendant can be required to establish certain defences. This I feel is a fair just and balanced legal doctrine.