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Those procedures which, at some future date, could constitute the operations resulting in the cloning of a human being are defined as a tool. As humans have been using tools for some two million years, sets of rules or ethics have been devised to make sure that tools are used to promote the maximum benefit and cause the minimum harm. It would, therefore, seem appropriate to consider the human cloning process as one such tool and approach the ethical issues which might govern its use in a manner which has been informed by how we have approached similar instances of new tool development. In this we can be guided by a range of thought experiments and practical considerations. When we combine these two facilities we may conclude that there are some benefits to be gained by the use of the human cloning tool, while there are other aspects of the use of the tool that need to be carefully controlled and regulated. An outline of a pragmatic approach to the initial uses of the tool is described.  相似文献   
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Three experiments examined the effects of constant vs. varied input of letter strings on recall, and then examined the effects of such training in the learning of new lists of letter strings. Letter strings were constructed from pairs of trigrams spatially grouped, and were presented either consistently or with different spatial groupings on successive presentations. In Experiments I and II, varied input produced substantially greater recall than constant input. When transferred to a new list of letter strings, containing either the same general structure or a new scrambled structure, recall of the second list was determined primarily by conditions of first-list input, and unaffected by second-list structure. Although the "variability effect" did not appear in the training phase of Experiment III, Varied input led subjects to regroup or integrate the letter sequences more frequently and produced similar transfer effects as in Experiments I and II.  相似文献   
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