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Pieces of circumstantial evidence don't have arbitrary proof thresholds. Their value depends on the logical alternatives.
Moreover, a "piece of evidence" is not an object, but a reasoning. Evidence is not something physical, it's something logical, it's a piece of reasoning. Not a finding itself, but an inference that links a finding to other findings. It's developed from factual findings, but does not equate to any specific finding.
The statements in the quoted post are the opinions of Machiavelli, and are totally contrary to Italian procedural law. Machiavelli claims to be presenting Italian law, but that is not at all true.
For example, Italian procedural law, CPP Article 192, Evaluation of evidence, states:
1. The judge shall evaluate evidence specifying the results reached and the criteria adopted in the grounds of the judgment.
2. The existence of a fact cannot be inferred from circumstantial evidence unless such evidence is serious, precise and consistent.
3. The statements made by either the co-accused charged with the same offense or a person accused in joined proceedings according to Article 12 shall be corroborated by the other elements of evidence confirming their reliability.
4. The provisions of para. 3 shall apply also to the statements made by a person accused of an offence that is joined to the one being prosecuted, in the case referred to in Article 371, paragraph 2, letter b).
Note: Paragraphs 3 and 4 above would apply to statements by Rudy Guede.
Also consider CPP Article 191, Unlawfully gathered evidence:
1. Evidence gathered in violation of the prohibitions set by law shall not be used.
2. The exclusion of evidence may be declared also ex officio at any stage and instance of the proceedings.
Comments based on the above CPP Articles:
1. "Evidence" as a term or concept used in Italian courts - when they indeed follow Italian law - is essentially the same as that used in the courts of the US and other democratic states.
Evidence is defined as direct evidence, that is, the testimony of a person based on what that person has seen or heard during an event (and not as hearsay from another source) or as circumstantial evidence, which includes all objects or demonstrable facts admissible because they have been legally obtained, have probative value (are "serious"), have reliability including adherence to appropriate forensic science standards and precision - not mere compatibility (are "precise"), and are not contradictory (are "consistent").
Evidence is not a reasoning, inference, hypothesis, or theory. A "reasoning" or "grounds of the judgment" is a finding, conclusion, or judgment and the explanation for the motivation of the finding, conclusion, or judgment, and is, by Italian law, to be based on evidence, and developed by the process of reasoning. Inference is a reasoning process to derive a finding, conclusion, or judgment from evidence.
2. A "finding" is, for the US, a determination of "fact" - a conclusion based on fact - from other facts by a judge or jury. The "finding" based on facts is to be distinguished from a "conclusion of law" which is the opinion or interpretation of a judge of what the law is based solely on consideration of existing laws.